When a car is going at a constant velocity down the freeway, is the net force zero or not zero assuming no friction?

Zero

Explanation:

The net force acting on an object moving with constant velocity is zero. This is an accordance with the Newton's first law of motion.

Newton's first law of motion states that a body will continue in its state of rest or constant motion unless it is acted upon by an external force.

Since this body is moving with uniform motion, it is not accelerating. When a body is acceleration, the net force on it is not zero.

But this one is moving with uniform motion. The net force on the body is balanced and zero.

d. air pressure decreases as altitude increases.