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The protocol data unit (PDU) for the application layer in the Internet stack is:

1. Segment
2. Datagram
3. Message
4. Frame

(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

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The answer is Message but I have a doubt. If datagram is related to UDP datagram or IP datagram?
reshown
I think datagram is pdu of network layer and not Transport layer, as read from kurose and ross 6th edition pg 51. Why is it that there are so many upvotes for this answer? Am I missing something??
The datagram is PDU at the transport layer. When UDP protocol is used at transport layer, a unit is called as a datagram in the connection-less transfer (while a unit is called as a segment in case of TCP).

Packet is the PDU at Network layer.
@coolsid

The transport layer PDU is the segment or the datagram.

Source: https://en.wikipedia.org/wiki/Protocol_data_unit#OSI_model
PDU for the presentation layer is messages.

PDU for transport layer is segment.

PDU for network layer is packet or datagram.

PDU for datalink layer is frame .

PDU for physical layer are bit streams .
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Option-C
In Netwok layer PDU is called as Datagram.

In transport layer ,PDU is called as Segment.

Whereas In the application layer the PDU is called as the Message.
PDU for application layer is message.
For transport layer is segment.
For network layer is packet or datagram.
For datalink layer is frame and for physical layer is bit streams.
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## Layer

Segment Transport Layer
Datagram Transport Layer
Message Application Layer
Frame  Data Link Layer

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Datagram -》transport layer or  network layer or udp?
Datagram $\rightarrow$ UDP

Segment $\rightarrow$ TCP
message is the correct at application layer the user directly abble open it and use it
Answer is C – Message

Packet is a generic name for all layers.

Message – AL

Segment – TL

Datagram – NL

Frame – DL

Single PDU (IPDU) – PL
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Option C) Message , In application Layer PDU is message.
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appropiate answer is c
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