In protocol communication, what does a negative acknowledgment (NAK) indicate?

• A. The previous message was not received correctly.
• B. The connection has been closed.
• C. The next message should be sent immediately without checks.
• D. The previous message was acknowledged successfully.

Answer: (A)

A negative acknowledgment signals that the receiver detected an error in the previous message and cannot accept it as correct. When a NAK is sent, the sender is prompted to retransmit or otherwise correct the data, typically after a checksum, CRC, or parity check indicates corruption or a missing piece. This is different from a successful acknowledgment, which means the message was received correctly, and from signals that the connection is closing or that no error checking occurred. In practice, if a frame fails the integrity check, the receiver replies with a NAK so the sender can resend that specific data.