Explanation:

HV (High Voltage) transmission lines are used to transmit electrical power over long distances at high voltages. These transmission lines are subjected to high electrical stress and mechanical loads due to the high voltage and weight of the conductors. To ensure the safe and reliable operation of the transmission line, insulators are used to support the conductors and prevent electrical current from flowing into the supporting structures.

The correct answer for the type of insulators used in HV transmission lines is option C - both pin type insulators and suspension insulators. Let's understand the characteristics and applications of these two types of insulators.

1. Pin type insulators:
- Pin type insulators are made of porcelain or glass and are designed to support the conductors on the transmission towers.
- They have a cylindrical shape with a flange at the bottom that is fixed to the tower structure.
- The conductor is held in place by a metal pin that passes through the insulator.
- Pin type insulators are typically used in lower voltage transmission lines.
- They provide good electrical insulation and mechanical strength to support the conductor.
- However, they are not suitable for very high voltage transmission lines due to limitations in their mechanical strength.

2. Suspension insulators:
- Suspension insulators are also made of porcelain or glass and are used to support the conductors in HV transmission lines.
- They have a string of insulator units connected together with metal fittings.
- The conductor is suspended from the tower structure using these insulator strings.
- Suspension insulators are designed to withstand higher electrical and mechanical loads compared to pin type insulators.
- They have a longer creepage distance, which helps in preventing flashovers and maintaining electrical insulation.
- Suspension insulators are used in HV transmission lines where the voltage levels are higher and the mechanical loads are greater.

In summary, HV transmission lines use both pin type insulators and suspension insulators. Pin type insulators are used in lower voltage lines, while suspension insulators are used in higher voltage lines to provide better electrical insulation and mechanical strength. The choice of insulator type depends on the voltage level and mechanical requirements of the transmission line.