Series capacitors on transmission lines are of little use when the load VAR requirement is large, small, or fluctuating. The correct answer is option 'D'.

Explanation:
Series capacitors are often used on transmission lines to improve the line's voltage profile and increase its power transfer capability. However, their effectiveness depends on the characteristics of the load and the power system.

1. Load VAR Requirement is Large:
When the load VAR requirement is large, it means that the load is consuming a significant amount of reactive power. In this case, the series capacitors may not be able to compensate for the reactive power demand of the load. Series capacitors provide capacitive reactive power, which is opposite in nature to the inductive reactive power consumed by the load. Therefore, if the load requires a large amount of reactive power, the series capacitors may not be sufficient to meet the load's VAR requirement.

2. Load VAR Requirement is Small:
On the other hand, when the load VAR requirement is small, it means that the load is consuming a relatively low amount of reactive power. In this case, the series capacitors may provide excessive capacitive reactive power, leading to overvoltage issues on the transmission line. Overvoltage can cause insulation breakdown, equipment damage, and system instability. Therefore, in situations where the load VAR requirement is small, series capacitors may not be necessary and can even be detrimental to the power system's stability.

3. Load VAR Requirement is Fluctuating:
If the load VAR requirement is fluctuating, it means that the reactive power demand of the load is constantly changing. Series capacitors have a fixed capacitive reactance and cannot adapt to the varying load conditions. They may be ineffective in compensating for the changing reactive power demand, leading to voltage fluctuations and instability in the power system. In such cases, other reactive power compensation techniques like shunt capacitors or static VAR compensators (SVCs) may be more suitable.

In summary, series capacitors on transmission lines are of little use when the load VAR requirement is large, small, or fluctuating. They may not be able to provide sufficient reactive power compensation or may cause overvoltage issues and system instability. Other reactive power compensation methods should be considered in such situations.