If a letter clearly states the requested action, its purpose, and the addressee, but gives no deadline, what essential information will the receiver not know?

Correct answer: By when the action should be completed

A deadline tells the receiver when the requested action must be completed. The question states that the action, purpose, and addressee are already clear, so those cannot be the missing elements. Therefore A is correct. Without a deadline, the receiver may delay the task or make different assumptions about urgency.