When the sender and receiver of an email are on the same system, we need only _____ user agents.

Correct option is A

When both the sender and receiver of an email are on the same system, only two user agents are needed—one for the sender and one for the receiver—to manage the communication process.
(b) Three: This is incorrect because three user agents would imply an additional intermediary, which is not needed when both sender and receiver are on the same system.
(c) Four: This is incorrect because four user agents would involve an even more complex scenario, unnecessary for same-system communication.
(d) Six: This is incorrect as six user agents are excessive and not required for communication between users on the same system.