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.