The correct answer is option 'C': Star delta starter operates at 58% of full voltage.

Explanation:

1. Introduction to starters:
- Starters are used to start and stop the motors in electrical systems. They provide the necessary control and protection to the motor.
- Two commonly used starters are Direct-On-Line (D.O.L.) starter and Star Delta starter.

2. Direct-On-Line (D.O.L.) starter:
- In a D.O.L. starter, the motor is directly connected to the power supply at full voltage during the starting process.
- When the motor is started, it draws a high starting current, which can cause voltage drop and affect other devices connected to the same power supply.
- The high starting current can also cause mechanical stress and damage to the motor windings.

3. Star Delta starter:
- A star delta starter is a reduced voltage starter that reduces the starting current and torque of the motor during the starting process.
- It consists of two contactors and a timer. The motor is initially connected in a star configuration, where each motor phase is connected to one end of the respective contactor coil, and the other end of the contactor coils is connected together.
- During the starting process, the motor operates at a reduced voltage of approximately 58% of the full voltage.
- After a predefined time, the contactors switch the motor connections to a delta configuration, where each motor phase is connected directly to the power supply.
- The switch from star to delta configuration is done to provide full voltage to the motor and achieve full torque for normal operation.

4. Advantages of star delta starter:
- Reduced starting current: By operating at a reduced voltage during the starting process, the star delta starter reduces the inrush current, which helps prevent voltage drop and mechanical stress on the motor windings.
- Reduced torque: The reduced voltage also reduces the torque during the starting process, which can be beneficial for applications where a high starting torque is not required.
- Cost-effective: The star delta starter is a cost-effective solution compared to other reduced voltage starters like autotransformer starters or variable frequency drives.

In conclusion, the star delta starter operates at approximately 58% of the full voltage during the starting process. This reduced voltage helps in reducing the starting current and torque, providing a smoother and controlled start to the motor.