Consider a parachutist that has reached terminal velocity. which of the following is true? a.) the acceleration of the parachutist equals gravity b.) at terminal velocity there is no net force c.) at terminal velocity the parachutist is no longer falling d.) at terminal velocity the force are unbalanced

The correct answer is:
B.) At terminal velocity there is no net force 

In fact, when the parachutist reaches the terminal velocity, his velocity does not change any more. It means that the acceleration acting on the parachutist is zero, and for Newton's second law, this means the net force acting on him is zero:

because the acceleration is zero: a=0. 
This also means that the two relevant forces acting on the parachutist (gravity, downward, and air resistance, upward) are balanced to produce a net force equal to zero.

A. to determine if humans can explore mars safely