Humidity has no effect on controlling a static charge on the surface of which of the following?

Humidity primarily impacts the static charge on surfaces by providing moisture in the air, which can lead to increased conductivity. This is particularly relevant in solid and plastic materials, where moisture can make the surface more conductive and allow static charges to dissipate more easily.

However, liquids are inherently conductive due to the presence of mobile charge carriers (ions) in them. This means that static charges do not accumulate on the surface of liquids in the same way as they do on solids or plastics. Consequently, the effect of humidity would be negligible for controlling static charge on the surface of liquids.

By contrast, solids, gases, and plastics can all accumulate static charges, and their interaction with humidity plays a significant role in how those charges are managed. Therefore, the correct answer is that humidity has no effect on controlling static charge on the surface of liquids, as they are already inherently conductive.