Final answer:
Airfields are controlled by air traffic control towers and designated as Class B, C, or D airspace during operational times of the control services. Class B is for busy commercial airports, Class C for moderate traffic airports, and Class D for smaller airports with varying traffic.
Step-by-step explanation:
The periods of time that an airfield is being controlled by an air traffic control tower and are designated as Class B, C, or D airspace correspond to the times when an air traffic control service is operational within that airspace. Specifically, Class B airspace is generally around the nation's busiest airports to ensure a high level of control for commercial operations. Class C airspace is around airports that have a moderate level of air traffic, which includes a mix of commercial, cargo, and general aviation activities.
Finally, Class D airspace is found around smaller airports with an operating control tower, serving a mix of student, general aviation, and some commercial flights.
These classes of airspace have specific operational requirements, including communication protocols and clearance mandates for pilots. Out of the given choices, Class A airspace does not pertain to this description, as it's typically from 18,000 feet mean sea level (MSL) up to and including Flight Level 600 (approximately 60,000 feet MSL) and overlying the waters within 12 nautical miles off the coast of the 48 contiguous states and Alaska, requiring all aircraft to operate under Instrument Flight Rules (IFR).