Which of the following observations indicates that an object is falling at terminal velocity?
a. the object has mass.
b. the object’s speed is greater than zero.
c. the object’s speed continues to increase.
d. there are no net forces acting on the object.

 D

Explanation:

When an object falls gravity is pulling down on it and is picking up speed, but as it gains speed air resistance becomes a faster. Air resistance increases with speed. And that force keeps it from accelerating eventually the object will pick up speed such that the force due to air resistance will keep it from getting any more speed at that point force due to air resistance is equal to its weight (mg) and the net force is equal to zero so it won’t accelerate any more at that point it is said to be moving in terminal velocity.

When an object has reached terminal velocity, it will have a constant velocity

this means that the temperature of both substances will be equal

there are no contact forces mentioned on the list of choices that you've provided.