The incorrect option regarding action-reaction pair is option D, which states that the sum of action and reaction on/by a body is zero. Let's understand why this option is incorrect.

Action and Reaction Forces
- According to Newton's third law of motion, for every action, there is an equal and opposite reaction. This means that whenever a force is exerted on an object, the object exerts a force of the same magnitude but in the opposite direction on the source of the force.

Equal and Opposite Forces
- Option A states that action and reaction forces are equal and opposite. This is correct because the magnitude of the force exerted by the object on the source is equal to the magnitude of the force exerted by the source on the object. Additionally, the direction of these forces is opposite to each other.

Act on Two Different Bodies
- Option B states that action and reaction forces act on two different bodies. This is also correct because the action force is exerted by one body on another, while the reaction force is exerted by the second body on the first. These forces always act on different objects.

Act Only Between Bodies in Contact
- Option C states that action and reaction forces act only between bodies in contact. This is incorrect because action and reaction forces can act even when the bodies are not in direct contact. For example, when you push a book on a table, the action force is exerted by your hand on the book, and the reaction force is exerted by the book on your hand. Even though the book and your hand are not in direct contact, action and reaction forces still occur.

Explanation of Option D
- Option D states that the sum of action and reaction on/by a body is zero. This is an incorrect statement. The action and reaction forces act on different bodies and have equal magnitudes but opposite directions. Therefore, their sum is not zero. The action force and the reaction force are separate forces that act on different objects.

To summarize, the incorrect option regarding action-reaction pairs is option D, which states that the sum of action and reaction on/by a body is zero. This is incorrect because action and reaction forces are separate forces that act on different bodies and have equal magnitudes but opposite directions.