Final answer:
The reason for AM = BM when M is the midpoint of AB is the Definition of Midpoint.
Step-by-step explanation:
Definition of Midpoint: If M is the midpoint of AB, it means that M is the point that divides the segment AB into two equal parts. So, AM and BM are equal in length.
Example: Suppose we have a line segment AB with a length of 10 units. If M is the midpoint, then the distance from A to M would be 5 units, and the distance from B to M would also be 5 units.
Based on this definition, we can conclude that the reason for AM = BM when M is the midpoint of AB is Option A: Definition of Midpoint.