What is the first action the Network Layer must take with a received frame?

The first action that the Network Layer must take with a received frame is to decapsulate the packet. When a frame is received, it typically contains not only the payload but also the data link layer header and trailer. Decapsulation involves removing this data link layer encapsulation to extract the packet contained within.

This process allows the Network Layer to access the packet's header, which includes important information such as source and destination IP addresses. Once the packet is extracted and the headers are analyzed, the Network Layer can determine the next steps, such as routing or forwarding the packet to its intended destination.

In this scenario, the other options do not represent the first action taken by the Network Layer. Encrypting the packet is a process related to security and typically occurs at different layers or stages, rather than being a direct response to receiving a frame. Encapsulating the frame is a function performed by the Network Layer when sending data down to the Data Link Layer, and forwarding to the Data Link Layer is not the initial action; instead, it occurs after decapsulation when the packet is ready to be transmitted.