What occurs inside the pump volute due to insufficient pressure, causing cavitation?

Cavitation is a phenomenon that occurs when there is a significant drop in pressure within a pump system, particularly in the pump volute. When the pressure drops below the vapor pressure of the liquid being pumped, it leads to the formation of water vapor bubbles. These bubbles form as the liquid transitions from a liquid state to a vapor state due to the low pressure.

As the pump continues to operate, these vapor bubbles are carried along with the flow of the liquid and can subsequently collapse or implode when they move into higher pressure zones within the pump. This implosion creates shock waves that can lead to serious damage to the pump components, including erosion of the pump casing and impeller.

In the context of the question, the correct answer highlights the direct result of insufficient pressure within the pump volute, namely, the formation of water vapor bubbles, which is the primary indicator of cavitation occurring in the system.