Explanation:

A penstock is a long pipe or conduit used to carry water to a hydroelectric power plant. In this case, the penstock is 3000 meters long.

A pressure wave travels in the penstock with a velocity of 1500 m/s. This pressure wave is created when the turbine gates are closed.

When the turbine gates are closed, the water flow is stopped abruptly, causing a pressure wave to propagate through the penstock. The speed at which this pressure wave travels is determined by the elastic properties of the penstock material and the density of the water.

The period of time it takes for the turbine gates to be closed completely is 4.5 seconds. This means that the gates are closed uniformly and gradually over this period of time.

Types of Closure:

There are different types of closure for the turbine gates, depending on the speed at which they are closed. The options given are as follows:

a) Rapid closure: This means that the gates are closed in a very short period of time, causing a rapid change in the water flow. This can result in a high-pressure wave propagating through the penstock.

b) Slow closure: This means that the gates are closed gradually over a longer period of time. This allows the water flow to reduce gradually, resulting in a slower and less intense pressure wave.

c) Sudden closure: This means that the gates are closed abruptly, without any gradual reduction in water flow. This can result in a sudden and intense pressure wave.

d) Uniform closure: This means that the gates are closed uniformly and gradually over a specific period of time. This can result in a pressure wave that is neither too rapid nor too sudden.

Correct Answer:

The correct answer is option B: slow closure. This is because the gates are closed gradually over a period of 4.5 seconds. This allows the water flow to reduce gradually, resulting in a slower and less intense pressure wave propagating through the penstock.

By closing the gates slowly, the pressure wave is minimized, reducing the risk of damage to the penstock and other components of the hydroelectric power plant.