Which of the following is not a messaging mode in CoAP?

The correct answer is D. Piggyback.

CoAP is a lightweight, binary application-layer protocol for constrained devices and low-power networks. It is designed to be an efficient alternative to HTTP for machine-to-machine (M2M) and Internet of Things (IoT) applications.

CoAP defines four messaging modes:

• Confirmable (CON): A message is sent with a request for a response. The sender will wait for a response before continuing.
• Non-confirmable (NON): A message is sent without a request for a response. The sender will not wait for a response.
• ACK: A message is sent to acknowledge the receipt of a CON or NON message.
• PING: A message is sent to check the reachability of a peer.

Piggyback is not a messaging mode in CoAP. It is a technique that can be used to send a small amount of data with a larger message. For example, a device could send a PING message with a small amount of data, such as the device’s current temperature.

Here is a brief explanation of each option:

• Confirmable (CON): A message is sent with a request for a response. The sender will wait for a response before continuing. This is the most reliable messaging mode, but it is also the most expensive.
• Non-confirmable (NON): A message is sent without a request for a response. The sender will not wait for a response. This is the least reliable messaging mode, but it is also the most efficient.
• ACK: A message is sent to acknowledge the receipt of a CON or NON message. This is necessary to ensure that the sender knows that the message was received.
• PING: A message is sent to check the reachability of a peer. This is useful for keeping track of the availability of devices on a network.