At which OSI layer is a source IP address added to a PDU during the encapsulation process?

A source IP address is added to a Protocol Data Unit (PDU) during the encapsulation process at the Network layer, which is the third layer of the OSI model. The primary function of the Network layer is to facilitate the delivery of packets between devices across different networks. This layer is responsible for logical addressing, which includes the use of IP addresses.

When a packet is created at the Network layer, it encapsulates the segment coming from the Transport layer, and that's where the source IP address is added. This addressing allows routers to determine the optimal path for transmitting the data across interconnected networks.

In contrast, the Transport layer, which is focused on end-to-end communication between hosts, adds its own headers related to port numbers and session management but does not involve IP addresses. The Data link layer is mostly concerned with local area network addressing and the physical transmission of data frames over the network. Lastly, the Application layer is where end-user applications operate, and it does not deal with the specifics of addressing for data transmission. Therefore, the correct choice reflects the role of the Network layer in managing IP addressing necessary for routing packets effectively across the internet or other network systems.