Apassenger in the rear seat of a car moving at a steady speed is at rest relative to
a. the front seat of the car
b. the side of the road
c. a pedestrian on the corner ahead
d. the wheels of the car

"At rest" here means "not moving relative to." The answer depends to some extent on your definition of the items in question

The person is definitely not moving relative to the front seat of the car. So at rest relative to the front seat.

The person is moving relative to the side of the road, as long as you don't think of the side of the road as an infinite line (which seems reasonable). So not at rest relative to the side of the road.

The person is definitely moving relative to a pedestrian on the corner ahead -- the distance between them is decreasing. So not at rest relative to the person on the corner.

The wheels of the car are rotating, which is tricky. I confess that I would say the person is not at rest due to the wheel's rotation, but I'm not certain that's the true answer from a physics perspective. The person is not moving relative to the wheels of the car, if you think only of the distance between them and ignore the rotation. So I would say not at rest relative to the wheels. 

So I would answer the front seat of the car.

A passenger in the rear seat of the car could hang a book on the back
of the front seat.  According to him, whether or not the car is moving,
the book isn't, and he can read it whenever he feels like it.  Relative to
the front seat and the book hanging from it, the passenger in the back
seat is at rest.