What must you do before entering Class D airspace at an airport with an operating control tower?

Before entering Class D airspace at an airport with an operating control tower, it is essential to contact the tower. This requirement is in place to ensure safe and efficient operations in controlled airspace. Class D airspace typically extends from the surface up to a specified altitude around an airport, where air traffic control provides instructions and traffic advisories to pilots.

By contacting the tower, you establish communication with air traffic control, allowing them to provide necessary information regarding traffic, weather conditions, and any other pertinent details that may affect your flight. This communication is crucial for maintaining safety and situational awareness for all aircraft in the vicinity of the airport.

The other options, while relevant to aviation operations, do not directly address the requirements for entering Class D airspace. Entering the Mode C veil pertains to the necessity of having an operating transponder within certain airspace, and joining the airport traffic pattern is a procedure for managing arrivals and departures, but both are secondary to first establishing communication with the control tower.