What must you have before operating in Class B airspace?

To operate in Class B airspace, having a Mode C transponder is essential. Class B airspace is typically found around the busiest airports, where there is a high volume of air traffic. The requirement for a Mode C transponder, which provides altitude reporting information, is crucial for air traffic control to manage the airspace effectively and maintain safe separation between aircraft. This system allows for precise tracking of aircraft movements, enhancing safety for all aircraft operating in the congested environment of Class B airspace.

The other options, while important in various contexts, do not specifically fulfill the regulatory requirement to operate in this type of airspace. A flight instructor's presence is not mandated for compliance, as pilots can operate independently once they have met the necessary qualifications. An ATC clearance is needed upon entering Class B airspace, but that is not equivalent to a requirement to have a Mode C transponder. Similarly, while a weather briefing is an important part of flight planning and safety, it is not a specific requirement for operating in Class B airspace. The regulatory focus is squarely on the need for the transponder for safe airspace management.