Which designation indicates a trainer aircraft?

The designation that indicates a trainer aircraft is "Trainer." This classification is specifically used to signify aircraft designed for the primary purpose of training pilots. Trainer aircraft typically have features that facilitate the learning process, such as dual controls to allow instructors to take over in case of an emergency, simplified avionics for easier handling, and designs that make them more forgiving for novice pilots.

In contrast, the other designations serve different purposes; for example, research/experimental aircraft are used primarily for testing new technologies or concepts rather than pilot training. Utility aircraft are typically employed for various operational tasks like cargo transport, medical evacuation, or reconnaissance, emphasizing versatility rather than training. Weather aircraft, on the other hand, are specifically equipped for meteorological observations and studies. Understanding these distinctions helps clarify why "Trainer" is the correct designation for aircraft intended for pilot education.