Question

The earth attracts the apple with a force of 2N. If this is the force of action, what would be the force of reaction and who exerts this force? Do we observe this force in nature, if not, why?

Answer 1

Reaction force is always same in magnitude of actopposite direction.

Therefore
Reaction force = 1.5 N
Apple exerts this reaction force and on earth this reaction force acts.

Answer 2

By the 3rd law, the force should be mutual.

But you need to remember that "observing" force in nature implies a none zero force.

The resultant vector should not be zero. Reaction force is always same in magnitude of act opposite direction.

Therefore Reaction force = 1.5 N.

Apple exerts the reaction and adding with force on earth.