Action and Reaction are equal in magnitude and opposite in direction. Then, Why do they not balance each other?​

Newton's third law of motion states that every action has an equal and opposite reaction. This indicates that forces always act in pairs. Reaction forces are equal and opposite, but they are not balanced forces because they act on different objects so they don't cancel each other out.

v=ir = 3x15 =45 v

the answer is b, have negligible mass