The correct answer is option 2: a non-zero constant.

When a car undergoes uniform circular motion, it means that it is moving in a circular path at a constant speed. Even though the speed is constant, the car is changing direction continuously. This change in direction requires a force to act on the car, which in turn causes an acceleration.

The acceleration of the car in uniform circular motion is directed towards the center of the circle and is known as centripetal acceleration. This acceleration is always non-zero because the car is constantly changing its direction.

Furthermore, the centripetal acceleration is constant because the speed of the car remains the same throughout the circular path. The centripetal acceleration is given by the formula a = v^2 / r, where v is the velocity (speed) of the car and r is the radius of the circular path. Since v is constant and r is also constant for the car in uniform circular motion, the acceleration remains constant as well.

Therefore, option 2: a non-zero constant, is the correct choice.