The correct answer is option 'B': speed control is required.

Explanation:
A wound rotor motor, also known as a slip ring motor, is a type of induction motor where the rotor windings are externally connected to resistors through slip rings. This configuration allows for greater control over the motor's speed and torque characteristics.

Below are the reasons why a wound rotor motor is mainly used in applications where speed control is required:

1. Variable Speed Operation: The wound rotor motor is designed to operate at variable speeds. By varying the resistance connected to the rotor windings through the slip rings, the speed of the motor can be adjusted in a wide range. This makes it suitable for applications where precise speed control is required, such as in industrial processes, machine tools, and conveyor systems.

2. Starting Torque Control: The wound rotor motor provides a higher starting torque compared to a squirrel cage induction motor. This is achieved by initially connecting external resistors to the rotor windings, which limits the starting current and provides a higher torque during startup. The ability to control the starting torque is essential in applications where the motor needs to start under heavy loads or in situations with a high inertia.

3. Smooth Acceleration and Deceleration: The presence of external resistors in the rotor circuit allows for smooth acceleration and deceleration of the motor. By gradually reducing the resistance, the motor can be brought up to speed without sudden jerks or shocks. This is particularly beneficial in applications that require precise control over the motor's acceleration and deceleration, such as elevators, cranes, and escalators.

4. Energy Efficiency: The wound rotor motor can achieve higher energy efficiency compared to other types of motors, especially when used in conjunction with external variable resistors. By adjusting the resistance in the rotor circuit, the motor can operate at its optimal efficiency point for a given load. This helps in reducing power consumption and improving overall energy efficiency.

In conclusion, a wound rotor motor is mainly used in applications where speed control is required due to its ability to provide variable speed operation, starting torque control, smooth acceleration and deceleration, and improved energy efficiency.

A