Lightning arresters, also known as surge arresters or lightning protection devices, are used in power systems to protect electrical equipment against power frequency overvoltages. Power frequency overvoltages can occur due to various reasons, such as switching operations, faults in the power system, or lightning strikes.

The correct answer is option 'C' because lightning arresters are primarily designed to protect against power frequency overvoltages. These overvoltages can be caused by lightning strikes indirectly affecting the power system.

Here is a detailed explanation of why lightning arresters are used to protect against power frequency overvoltages:

1. Lightning Strokes and Overvoltages: Lightning is a natural phenomenon that can cause direct strokes of lightning on power lines or nearby structures. These direct strokes can lead to extremely high voltages and currents that can damage electrical equipment. However, lightning can also cause indirect strokes, which induce high voltages and currents in power lines due to electromagnetic coupling. These indirect strokes can result in overvoltages in the power system, which can damage or disrupt the operation of electrical equipment.

2. Purpose of Lightning Arresters: The main purpose of lightning arresters is to protect against overvoltages caused by lightning strikes. When a lightning strike occurs, the arrester provides a low-impedance path to divert the excessive current safely to the ground. By doing so, the arrester limits the voltage rise and protects the connected electrical equipment from the damaging effects of overvoltages.

3. Operation of Lightning Arresters: Lightning arresters consist of a series of metal oxide varistors (MOV) or silicon carbide varistors (SIC) connected in parallel with the equipment to be protected. These varistors have non-linear voltage-current characteristics, which means they have a high resistance under normal operating conditions but a low resistance when the voltage exceeds a certain threshold. When a power frequency overvoltage occurs, the varistor conducts, providing a low-impedance path to the ground and limiting the voltage across the protected equipment.

4. Power Frequency Overvoltages: Power frequency overvoltages can occur due to various reasons, such as lightning strikes, switching operations, or faults in the power system. Lightning arresters are specifically designed to protect against overvoltages caused by lightning strikes. They are not primarily intended to protect against direct strokes of lightning or overcurrents due to lightning, as mentioned in options 'A' and 'D'.

In conclusion, lightning arresters are used in power systems to protect electrical equipment against power frequency overvoltages. These overvoltages can occur due to lightning strikes indirectly affecting the power system. Lightning arresters provide a low-impedance path to divert excessive current safely to the ground, limiting the voltage rise and protecting the connected equipment.