CD distance represents the distance from point d to ab.
We are given to choose the correct option that represents the distance from point D to AB.
We know that the distance between a point and a line is the length of the perpendicular drawn from the point to the line.
Cause the CD is the mid point of triangle ADB which divides triangle in two half if you look carefully you can understand the diagram
So, here CD will be the required distance from point D to AB, because CD is perpendicular from point D to AB.
Thus, (A) is the correct option.