Which of the following describes a "clearance" given by air traffic control?

A clearance given by air traffic control is essentially a permission granted to a pilot to take off or land at an airport. This clearance is a critical component of air traffic operations, ensuring that aircraft operate safely and efficiently in the controlled airspace. When pilots receive a clearance, it indicates that the air traffic controller has determined it is safe for the aircraft to proceed with either departure or arrival based on current traffic conditions and runway availability.

Regarding the other options, while starting engines, maintaining altitude and speed, or changing flight paths could involve aspects of communication with air traffic control, they do not specifically define the unique and formal concept of a "clearance." A clearance is distinct in that it represents explicit permission for critical phases of flight linked directly to the operations of taking off from or landing at an airport.