Vmd occurs where a tangent from the origin touches the power-required curve.
Doesn't that assume engine efficiency being constant?
Traditionally, the best range speed is equal to Vbg (best glide speed) but this assumes constant engine efficiency, AIUI. In practice, Vbr (best range speed) occurs at a slightly higher power setting.
But you need to be careful. The exam questions are written assuming you have studied the right books - not necessarily for the most accurate knowledge.