Aphysics student in a hot air balloon ascends vertically at constant speed. consider the following four forces that arise in this situation:
f1 = the weight of the hot air balloon f3 = the force of the student pulling on the earth
f2 = the weight of the student f4 = the force of the hot air balloon pulling on the student
which two forces form an "action-reaction" pair that obeys newton's third law?

F2 and F3 forms action-reaction pair

Explanation:

In accordance to Newton's third law of motion, action-reaction pairs are formed.

The law is based on the fact that for every action there exist a force that acts opposite and equal to the action force resulting in action reaction pair as they act on different object leading to the motion.

Here, the hot air balloon is in vertically upward motion at constant speed, the speed remains same resulting in no acceleration and also the weight of the student exerts a force opposed by the force exerted by the balloon