Correct Answer - Option 2 : Pressure approaches vapour pressure
Cavitation occurs when the liquid in a pipe turns to a vapor at low pressure. When Cavitation takes place, air bubbles are formed when this low pressure reduces to the vapor pressure of liquid. As the liquid passes from low pressure to high pressure region, the air bubbles explode. This creates a shockwave that hits the pipe material and leading to bursting of pipe.
Prevention of Cavitation:
1. Lower the temperature.
2. Raise the liquid level at low pressure region.
3. Increase the diameter of pipe.
4. Use pipes in parallel.