Question

Forces contribute to the net force on a car rolling down a ramp

Answer 1

Well if you're asking for the forces, here ya go.

There are a ton of forces that act upon a car while it rolls down a hill, such as:

     -Gravity pushing down on the mass of the car
     -The ramp (or ground) pushing back up on the car at an angle (making it            move)
     -Friction in the form of the tires on the ramp and air resistance
     -Normal Force perpendicular to gravity on the ramp

Now the question doesn't tell me if the engine is powering the car or not, it only says it is "Rolling"

But if the engine were to be driving the wheels, then Engine Force will be applied at the angle of the ramp.