The OAuth 2.0 Asset Token Flow is Salesforce’s purpose-built answer for connected devices and IoT-style integrations. It is designed for assets that need to act on behalf of a physical device or asset record rather than a human user. That makes it the correct fit for sensors, GPS trackers, or smart devices that must securely post telemetry or create service actions in Salesforce. User-agent, web-server, or username-password flows are built around user identity or generic application access, not around a registered device/asset relationship. The architectural advantage of the asset token model is that it ties the authorization context back to the asset, which is exactly what you want when the platform must know which device generated the event or maintenance alert. This is why option B is the best answer in Salesforce terms.

For fine-grained delegated access and long-lived sessions, the Authorization Code flow is the strongest mainstream OAuth choice. It supports user consent, server-side token exchange, and refresh-token issuance in the patterns Salesforce documents for confidential applications. Client Credentials is for app-to-app integration without a user context, and Implicit/User-Agent is less suitable for strong control and durable token management. Username-password is a special-case flow that trades off security and user experience. The reason this matters architecturally is that the authorization code pattern separates user authentication from token exchange and allows the client to keep secrets securely on the server side. That is why it remains the preferred model for robust delegated access to protected Salesforce resources. This is why option B is the best answer in Salesforce terms.