Which type of airspace requires a transponder to enter?

Class B airspace requires a transponder to enter because it encompasses the airspace around major airports where there are high volumes of air traffic. The transponder enhances the situational awareness of air traffic controllers by allowing them to identify and track aircraft within this busy environment. This is crucial for maintaining safety in Class B airspace, where separation between aircraft is essential due to the density of operations.

Additionally, while Class C airspace also requires a transponder, the question asks specifically about which airspace type requires it, with Class B airspace being the most restrictive and busy, making it a top priority for transponder usage. Class D and E airspaces have less stringent requirements regarding transponders, with Class D requiring communication but not necessarily a transponder, and Class E typically only requiring a transponder under certain conditions.