The correct answer is (d), seats.
The chassis is the frame or skeleton of a vehicle. It is the main structure that supports the weight of the vehicle and its components. The chassis also provides a mounting point for the engine, transmission, suspension, and other components.
The wheels are attached to the axles, which are part of the suspension system. The differential is a part of the transmission system that allows the wheels to rotate at different speeds when the vehicle is turning. The gear box is a part of the transmission system that changes the speed and torque of the engine.
The seats are not part of the chassis. They are part of the interior of the vehicle. The seats provide a place for the driver and passengers to sit.