The aircraft does have minimum drag at the speed of Vmd on the total drag curve ! But you are missing the point of the question in that if the aircraft is flying straight and level it must be producing lift to support the weight of the aircraft. That means at the speed of Vmd the aircraft is producing lift at minimum drag ie Lift/Drag ratio max or Cl/Cd max.

This is a very important point of reference on the drag curve. Since alpha also changes with change of speed at Vmd the aircraft is also operating at a particular alpha which is most efficient. This is generally considered to be about 4 degrees. Fly fast alpha decreases but drag increases for the same lift, fly slower alpha must be increased to maintain lift but drag again increases.