The correct answer is A. Field control.
Field control is the preferred speed control method for constant torque drives because it provides a wide range of speed control with good efficiency. By increasing or decreasing the field current, the speed of the motor can be controlled without affecting the torque. This makes field control ideal for applications where a constant torque is required, such as conveyors, hoists, and cranes.
Armature voltage control is another method that can be used to control the speed of a DC motor. However, armature voltage control does not provide as wide a range of speed control as field control, and it can also lead to decreased efficiency.
Shunt armature control is a third method that can be used to control the speed of a DC motor. However, shunt armature control is not as common as field control or armature voltage control. This is because shunt armature control can lead to increased heating of the motor, and it can also be difficult to control the speed accurately.
Mechanical loading system is not a method that can be used to control the speed of a DC motor. A mechanical loading system is a device that is used to apply a load to a motor in order to control its speed. However, a mechanical loading system is not as precise or efficient as the other methods that are listed above.
In conclusion, field control is the preferred speed control method for constant torque drives because it provides a wide range of speed control with good efficiency.