db.currentOp() is the mongosh helper used to inspect operations currently executing on a MongoDB server. Passing true requests a broad view that includes all in-progress operations as well as additional idle/system entries, so it can be used when the administrator wants to see the complete current-operation picture. The database-command form, db.adminCommand({ currentOp: true }), invokes the corresponding administrative command and is also valid for obtaining in-progress operation information. Option A adds a filter restricting the result to operations whose op field is " command " ; it therefore does not represent all operation types. Option B is not valid currentOp command syntax, and option E uses an all field that is not the documented special form for this helper. In modern MongoDB administration, the $currentOp aggregation stage is generally preferred for richer filtering and aggregation, but the question is testing the traditional helper and command interfaces. Therefore, C and D are the two valid choices among those presented.

Study Guide reference/topic:Performance Monitoring and Tuning - current operations, currentOp, and administrative diagnostics.

===========

db.collection.getIndexes() is the mongosh method used to return the definitions of all indexes that currently exist on a collection. The result is an array of index specification documents that can show fields such as the index name, key pattern, index version, uniqueness, partial-filter settings, sparse configuration, collation, and whether the index is hidden. This makes the method useful for index audits, troubleshooting, and confirming the physical indexes available to the query planner. Option A is not a documented method for enumerating collection indexes. Option B resembles the $indexStats aggregation stage, which reports usage statistics for indexes, but db.collection.indexStats() is not the method used to list full index definitions. Option C, findIndexes(), is also not the documented mongosh collection helper. Administrators should distinguish between listing index definitions, examining index usage, and inspecting an individual query ' s winning plan: getIndexes() addresses the first requirement, $indexStats addresses usage metrics, and explain() addresses query-planner behavior. Therefore option D is the correct command for listing all indexes on the collection.

Study Guide reference/topic:Indexing and Query Optimization - index administration, index metadata, and db.collection.getIndexes().

===========

Adding replica-set members addresses availability and recovery objectives. A normal secondary maintains a replicated copy of the primary ' s data and can be elected primary if the current primary becomes unavailable, thereby improving service availability compared with a single-node deployment. That directly supports option D. A delayed member intentionally applies oplog entries later than the primary, preserving an older logical state of the dataset for a configured delay period. This can help recover from operator mistakes such as an accidental destructive update or drop operation because the administrator may be able to access the delayed copy before the damaging operation is applied there. That supports option A. A delayed member is not a substitute for long-term archival or a formal backup-retention policy; its recoverable history is constrained by the delay configuration and oplog window, so B is not the intended objective. Option C describes sharding, which partitions data across shards rather than simply replicating it. Encryption at rest is configured independently and is unrelated to adding secondaries or delayed members. Therefore, A and D are the two reasons that align with the proposed replication design.

Study Guide reference/topic:Cluster Reliability and Data Resilience - replica sets, delayed members, failover, and operator-error recovery.

===========

MongoDB describes oplog operations as idempotent. An idempotent operation can be reapplied without causing an unintended cumulative change beyond the intended final state. This property is important for replication and recovery because secondary members copy and apply oplog entries, and operational processes may need to reason safely about replay. MongoDB can represent client write activity in oplog-friendly forms specifically to preserve appropriate replication semantics. Natural ordering is relevant to the capped collection structure of the oplog, but it is not the attribute identified by the question as providing safe repeatable application behavior. Concurrency control governs simultaneous operations and storage-engine coordination rather than the defining form of oplog entries. Flow control is a replica-set mechanism that can regulate the primary ' s write throughput when replication lag threatens majority-commit behavior; it does not make individual oplog entries safe to replay. The idempotent nature of oplog operations supports deterministic replication and contributes to consistent secondary state as operations are applied in sequence. Therefore the correct attribute among the options is Idempotent, making D correct.

Study Guide reference/topic:Cluster Reliability and Data Resilience - the oplog, idempotent replication operations, replay, and consistency.

===========