Which of the following is not a required preflight action?

The correct answer is C, which pertains to performing a full engine overhaul. This action is not a required preflight action for a flight. Preflight actions are typically focused on ensuring that the aircraft is in a condition for safe operation immediately prior to a flight. Required preflight actions include assessing the weather conditions, checking the status of air traffic control (ATC) delays, and verifying relevant runway lengths to ensure that the aircraft can safely take off and land.

In contrast, a full engine overhaul is a maintenance procedure that is performed periodically on the aircraft to ensure reliability and safety over the long term, rather than as a prerequisite for each individual flight. Preflight actions, on the other hand, are about ensuring readiness for the specific flight at hand, including all operational aspects that could affect safety.