Question

Inertia keeps your vehicle moving until a force slows it down, like __________.

Answer 1

Certainly  'A'  and  'C'. 

And I don't see why  'B'  couldn't also exert a force against the car
that would slow it down. 

I'd say "all of the above".

Answer 2

Technically all 3 of those would be correct answers. But applying the brakes would probably be the correct answer if all of the above isn't a answer.