Which type of airspace is associated with less busy airports that have a control tower?

The correct choice is Class D airspace, which is specifically associated with airports that have a control tower but are typically less busy than those in Class B or Class C airspace. Class D airspace is designed to provide a controlled environment for aircraft operating around smaller airports. This type of airspace extends from the surface up to a specified upper limit, usually 2,500 feet above ground level, and is established to ensure safe and orderly traffic management in and out of these airspace regions.

Class B airspace is associated with the busiest airports that require a higher level of control due to significant air traffic, while Class C airspace includes airports with a control tower that handle a moderate volume of traffic, but still busier than those found in Class D. Class E airspace is generally used for airspace that does not fall under Class A, B, C, or D, and it often exists around airports without a control tower, or in areas where controlled airspace is not necessary.

Therefore, Class D airspace is the most applicable to less busy airports with a control tower, enabling effective coordination of aircraft movements while still accommodating general aviation in a safe manner.