Point o is between points m and n on line segment mn. o is the midpoint of line segment mn if a. om = nm b. on = nm c. on = om d. on + om = mn

The word midpoint refers to the point that divides the line segment into two equal parts. From the given above, for O to be the midpoint, the length of the segment OM should be equal to segment ON. Thus, the answer is letter  C. The fourth choice may also be correct but it does not guarantee that O is the midpoint. 

The answer maybbe c