Explanation:

Shrouding is a technique used in tube wells to prevent the entry of unwanted materials into the well and ensure the flow of clean water. It involves the use of a slotted type tubewell.

Slotted Type Tubewell:
A slotted type tubewell is a type of tube well that has slots or openings along its length. These slots allow water to enter the well while preventing the entry of sand, silt, and other materials. The slots are usually made of durable materials such as PVC or stainless steel.

Advantages of Slotted Type Tubewell:
- Prevents the entry of sand and other unwanted materials into the well, ensuring the flow of clean water.
- Allows water to enter the well from a larger area, increasing the efficiency of the tubewell.
- Provides better filtration and prevents clogging of the tubewell.
- Increases the lifespan of the tubewell by reducing wear and tear caused by the entry of abrasive materials.
- Easy to install and maintain.

Other Types of Tubewells:
a) Cavity Type Tubewell: This type of tubewell does not have slots or openings along its length. It relies on the natural permeability of the surrounding soil or rock to allow water to enter the well. It does not provide the same level of filtration and protection against unwanted materials as a slotted type tubewell.

c) Strainer Type Tubewell: This type of tubewell has a strainer or filter at the bottom to prevent the entry of sand and other materials. It is similar to a slotted type tubewell in terms of preventing the entry of unwanted materials, but it may not provide the same level of filtration and efficiency.

d) None of the above: This option is incorrect as the correct answer is option 'b', which is the slotted type tubewell.

In conclusion, shrouding is provided in a slotted type tubewell to prevent the entry of unwanted materials and ensure the flow of clean water. It is an effective and efficient method of filtration and protection for tube wells.