Which layer of the OSI model is responsible for managing user applications and services?

Correct answer: Application Layer

Option D is correct because Layer 7, the Application Layer, is the topmost layer of the OSI model and is responsible for providing network services directly to end-user applications, including protocols such as HTTP, FTP, SMTP, and DNS. Option A is incorrect because the Presentation Layer (Layer 6) handles data translation, encryption, and compression between application formats, not direct management of user applications and services. Option B is incorrect because the Session Layer (Layer 5) is responsible for establishing, managing, and terminating communication sessions between applications, not for serving user-facing application services. Option C is incorrect because the Transport Layer (Layer 4) handles end-to-end communication, segmentation, and reliability through protocols like TCP and UDP, operating below the application service level.