Which of the following equipment suffixes indicates that an aircraft has no transponder?

The correct answer is indicated by the suffix that denotes an aircraft without transponder capabilities. In aviation, suffixes attached to an aircraft's call sign provide essential information about its equipment, particularly concerning transponders and other capabilities. The suffix "/X" specifically indicates that the aircraft does not have a transponder at all, which is crucial information for air traffic controllers in managing and ensuring safety in non-radar environments.

Other suffixes have different meanings; for example, "/D" indicates a transponder with Mode C (which provides altitude reporting), "/U" signifies the presence of a transponder with unknown status, and "/M" is used for aircraft with Mode S transponders that may also have additional capabilities. Understanding these suffixes allows controllers to make informed decisions about separation and traffic management based on the available equipment of the aircraft in their airspace.