Which of the following is NOT a main type of aircraft structure?

The term "frame and panel" refers to a structural method that combines various components to create a rigid framework, often found in smaller aircraft or older designs. However, it does not represent a primary category of aircraft structure in the same way that the others do. Monocoque, semi-monocoque, and truss structures are recognized as fundamental types of aircraft design.

Monocoque is a design that relies on the skin of the aircraft to support most of the aerodynamic stresses, making it lightweight. Semi-monocoque structures add reinforcement in the form of frames and stringers to enhance strength and durability while still utilizing the skin for structural integrity. Truss structures consist of a framework built from interconnected rods and beams, which distribute loads effectively and provide strong support.

Because "frame and panel" does not characterize a distinct, recognized structural type like the others, it accurately fits as the answer to the question regarding which choice is not a main type of aircraft structure.