There's the acceleration of the car that provides a force and the normal force of the seat cushion which pushes upwards against the passenger
A vibrating stretched string has nodes or fixed points at each end. The string will vibrate in its fundamental frequency with just one anti node in the middle - this gives half a wave.

Rearranging for the wavelength



Therefore the longest wavelength standing wave that it can support is 14m
THAT'S EASY BRO! the answer is the Seat!