Which of the following actions is mandatory upon receiving altitude clearance from ATC?

When altitude clearance is received from air traffic control (ATC), the mandatory action is to read back the clearance acknowledging the altitude change. This procedure is essential for ensuring that both the pilot and ATC have a mutual understanding of the instructions given. The readback confirms that the pilot has correctly understood the clearance and allows ATC to catch any potential misunderstandings before the pilot begins the maneuver.

In aviation communication, clarity and accuracy are paramount for maintaining safety in the airspace. The readback serves as a critical communication checkpoint, reducing the risk of errors that could occur if the clearance was altered in any way by misunderstanding.

While there may be other actions that a pilot could take upon receiving clearance, such as initiating a descent or acknowledging clearance verbally, those do not satisfy the requirement of confirming the specific altitude change through a readback. Additionally, submitting an altitude change request is not appropriate once clearance has been granted; it is a request procedure applicable before receiving an altitude change.