What does a takeoff clearance authorize a pilot to do?

A takeoff clearance specifically grants a pilot the authority to depart the airport using the designated runway. This clearance is an essential part of air traffic control procedures, ensuring that the runway is clear and ready for the aircraft to take off safely. It confirms that all necessary safety checks and communication have been completed between the pilot and air traffic control, allowing for a timely and organized departure.

The other options, while related to aviation operations, do not correctly encapsulate the specific permission granted by a takeoff clearance. For instance, landing at a designated airport pertains to a different phase of flight known as landing clearance. Entering controlled airspace is governed by separate regulations and may require additional communication, depending on the airspace class. Completing a flight plan is an administrative task that occurs before takeoff and does not fall under the authorization given by a takeoff clearance. Therefore, the takeoff clearance is fundamentally focused on the action of departing the runway, making the first choice the correct answer.