The relative velocity between Car A and Car B is 0 m/s
Since Car A is heading East at 25 m/s, its velocity is u = + 25 m/s. (since it is moving in the positive x- direction)
Also, car B is also heading East at 25 m/s, its velocity is v = + 25 m/s. (since it is moving in the positive x- direction)
So, the relative velocity between car A and Car B is V = velocity of car A - velocity of car B.
So, V = u - v
V = + 25 m/s - (+ 25 m/s)
V = + 25 m/s - 25 m/s
V = 0 m/s
So, the relative velocity between Car A and Car B is 0 m/s
The passenger in Car A would describe Car B's motion as stationary.
Since Car A is heading East at 25 m/s and car B is also heading East at 25 m/s, both cars are moving at a velocity of + 25 m/s.
Since their relative velocity is 0 m/s, it would appear to a passenger in Car A that car B is stationary according to motion from car A's reference frame.
So, the passenger in Car A would describe Car B's motion as stationary.