ANS:D - is suddenly brought to rest by closing a valve.

Water hammer in pipelines results when the flowing fluid is suddenly brought to rest by closing a valve. Water hammer, also known as hydraulic shock, occurs in pipelines when there is a sudden change in the flow velocity or direction of a fluid, typically caused by the rapid closure of a valve. When a valve is closed suddenly, the fluid in the pipeline is forced to come to an abrupt stop, creating a pressure surge that propagates backward through the pipeline. This pressure surge can generate high-pressure spikes, shock waves, and vibrations within the pipeline, potentially leading to damage to pipes, fittings, and other system components. The severity of the water hammer effect depends on factors such as the velocity and mass of the flowing fluid, the length and diameter of the pipeline, and the speed at which the valve is closed. In summary, water hammer occurs when the flowing fluid is suddenly brought to rest by closing a valve, causing a pressure surge that can potentially damage the pipeline system.