Which airspace requires specific clearance before entry?

D class airspace requires specific clearance before entry because it is the highest level of controlled airspace, generally located from 18,000 feet up to and including 60,000 feet. In this airspace, all aircraft must operate under Instrument Flight Rules (IFR), and pilots must receive an Air Traffic Control (ATC) clearance before entering. This requirement ensures the safe and orderly flow of air traffic in a congested environment where aircraft are flying at high altitudes and speeds.

In contrast, other classes of airspace have different entry requirements. For instance, class G airspace is uncontrolled, and pilots can operate without clearance. Class E airspace may require clearance under certain conditions but is not as strict as class A. Class C airspace does require communication with ATC before entering, but it does not have the same strict IFR requirements as class A airspace. Thus, class A airspace stands out for its mandatory clearance requirement due to the complexities and safety measures needed at higher altitudes.