Which of the following activities occur at the Application Layer?

The Application Layer of the OSI model is where user interaction with software applications occurs. This layer is responsible for providing network services directly to end-users and allows various applications to communicate over the network.

Email handling, which is the activity identified in the correct answer, involves managing emails through protocols like SMTP, POP3, and IMAP. These protocols operate at the Application Layer to encode, transmit, and retrieve email messages, making this function critical for user communications.

The other activities, while important aspects of networking, do not take place at the Application Layer. Data routing, for instance, is handled at the Network Layer, where data packets are directed to their destination based on IP addresses. Security management might occur across multiple layers, but specific security measures related to data transport often reside in the Transport Layer and lower. Session establishment typically takes place at the Session Layer, where sessions between applications are managed but does not concern user-level applications directly.

Thus, the focus on email handling at the Application Layer clearly illustrates how it serves user applications by facilitating communication processes.