Which type of buffet will occur if a jet aeroplane slowly accelerates in level flight from its cruise speed in still air at high altitude?

The scenario describes a jet airplane that is accelerating in level flight at high altitude. As the jet accelerates, it approaches its transonic speed, which is the speed range close to the speed of sound. This transition can lead to Mach buffet, a condition caused by local airflow reaching transonic or supersonic speeds over certain areas of the aircraft, particularly the wings and control surfaces.

As the aircraft accelerates further, shock waves begin to form on the wings, resulting in increased drag and potential flow separation. This phenomenon generates vibrations and oscillations, known as Mach buffet, that can affect the aircraft's control and handling characteristics. Awareness of Mach buffet is crucial for pilots, as it indicates the need for speed management to maintain safe aerodynamic performance.

In contrast, low speed buffet typically occurs when an aircraft is flown at slow speeds, and flutter speed buffet relates to dynamic stability and control issues at specific speeds rather than accelerating from cruise. Accelerated stall buffet happens during sharp maneuvering while maintaining a high angle of attack and is not directly related to gradual acceleration at cruising altitude. Thus, the characteristics of the situation described align most closely with Mach buffet, making it the correct choice.