The major structural unit of a vehicle is its:

The frame of a vehicle serves as its major structural unit, providing the foundational support that holds the entire vehicle together. It is primarily responsible for bearing the weight of the vehicle, supporting the suspension system, and ensuring the structural integrity during collisions or impacts. The frame also helps to maintain the proper alignment of various vehicle components, facilitating effective handling and stability.

Other components like the firewall, engine, and passenger compartment play important roles in the overall functionality and safety of the vehicle, but they do not provide the same foundational support or structural framework as the frame does. The firewall is primarily a protective barrier designed to shield passengers from engine heat and potential fires, while the engine serves as the power source, and the passenger compartment is designed for occupant protection and comfort. Thus, the frame's role is central to understanding vehicle structure and safety in rescue operations.