Final answer:
The property that allows us to conclude that CM = MD, given that M is the midpoint of CD, is symmetry.
Step-by-step explanation:
The property that allows us to conclude that CM = MD, given that M is the midpoint of CD, is symmetry. When M is the midpoint of CD, it means that CM and MD are equal in length. Since M is equidistant from C and D, there is symmetry in the distances between M and C, and M and D. So, we can conclude that CM = MD.