The correct answer is: A. Voltage control method
The voltage control method is the least efficient method of speed control of a DC machine because it requires the use of a rheostat, which dissipates power as heat. The field control method and the armature control method are more efficient because they do not require the use of a rheostat.
In the voltage control method, the speed of the DC motor is controlled by varying the voltage applied to the armature. This method is simple and inexpensive, but it is not very efficient because the rheostat dissipates power as heat.
In the field control method, the speed of the DC motor is controlled by varying the field current. This method is more efficient than the voltage control method because the field current does not dissipate power as heat. However, it is more complex and expensive.
In the armature control method, the speed of the DC motor is controlled by varying the armature current. This method is the most efficient method of speed control, but it is also the most complex and expensive.
Therefore, the voltage control method is the least efficient method of speed control of a DC machine.