Final answer:
The first indication that an airport has a control tower is radio communication with air traffic control.
Step-by-step explanation:
The first indication that an airport has a control tower is D) Radio communication with air traffic control. The control tower is responsible for managing air traffic at the airport, and radio communication with air traffic control is essential for pilots to receive instructions and ensure safe operations. The presence of radio communication with air traffic control indicates that there is a control tower at the airport.