Explanation:

Suspension insulators are used in high voltage transmission lines to support the conductors and provide electrical insulation between the conductor and the supporting structure. They are typically made up of a string of insulator discs, which are connected in series.

Each disc of the suspension insulator is designed to withstand a certain voltage. However, due to various factors, the voltages across the discs may differ. One of the main factors that cause the voltages across the discs to be different is the shunt capacitance to ground.

Shunt Capacitance to Ground:
When a suspension insulator is installed, it is usually mounted on a supporting structure. This structure is typically grounded. Due to the presence of this grounding, there is a certain amount of capacitance between the insulator discs and the ground.

This capacitance forms a voltage divider network, where the voltage across each disc is determined by the ratio of the capacitance to ground to the total capacitance of the insulator string. Since the capacitance to ground can vary between the different discs, the voltages across the discs will also vary.

Series Capacitance:
While series capacitance can also affect the voltages across the insulator discs, it is not the main reason for the voltage differences. Series capacitance refers to the capacitance between the adjacent insulator discs. This capacitance can cause voltage division as well, but its effect is relatively small compared to the shunt capacitance to ground.

Surface Leakage Currents:
Surface leakage currents occur when there is a conductive path between the conductor and the insulator surface. These currents can cause voltage differences across the insulator discs, but their effect is usually negligible compared to the capacitance effects.

Therefore, the correct answer is option 'C' - Shunt capacitance to ground. The capacitance between the insulator discs and the ground causes voltage division, resulting in different voltages across the discs.