Final answer:
High pressure prevents water from boiling at the normal surface temperature in underground caverns.
Step-by-step explanation:
The correct answer is a. High pressure.
The high pressure in the underground caverns prevents water from boiling at the normal surface temperature. When water is under high pressure, its boiling point increases. This is because the pressure on the water molecules effectively pushes them closer together, making it more difficult for them to escape into the gas phase.
For example, in deep underground caverns where the pressure is very high, water can remain in its liquid state at temperatures above its normal boiling point at the surface.