Two cars having different weights are traveling on a level surface at different constant velocities. within the same time interval, greater force will always be required to stop a car that has the greater
a)weight
b)velocity
c)kinetic energy
d)momentum

I think you're saying that once you start pushing on the cars, you want to be able to stop each one in the same time. 

This is sneaky.  At first, I thought it must be both 'c' and 'd'.  But it's not
kinetic energy, for reasons I'm not ambitious enough to go into.
(And besides, there's no great honor awarded around here for explaining
why any given choice is NOT the answer.)

The answer is momentum.

Momentum is (mass x speed).  Change in momentum is (force x time).

No matter the weight (mass) or speed of the car, the one with the greater
momentum is always the one that will require the greater (force x time)
to stop it.  If the time is the same for any car, then more momentum
will always require more force.