The protocol data unit (PDU) for the application layer in the Internet stack is:
(C) Message is the answer.
For Application, Presentation and Session layers, the PDU is message
For Transport layer, PDU is segment for TCP and datagram for UDP
For Network layer, PDU is packet
For Datalink layer, PDU is frames
For physical layer, PDU is stream of bits
The tests are there but it ain't free. Cost is...