When a person jumps horizontally off of the end of an unanchored boat in a lake, the person goes forward and the boat goes backward. this is an example of

Newton's third law of motion.  

Explanation:

When a person jumps horizontally off of the end of an unanchored boat in a lake, the person goes forward and the boat goes backward. This is an example of Newton's third law of motion.

To an action, there is an equal reaction but also opposite. The action and reaction forces acts on two different objects. One object moves in forward direction while others moves in backward direction.

Hence, the given scenario is an example of Newton's third law of motion.