The correct answer is: C. Both A. and B.
The maximum permissible velocity of 1.5 m/sec is adopted in drains with beds of rocks and gravels, as well as in drains lined with stones. This is because these types of drains have a higher capacity to carry water than other types of drains, and they are less likely to clog.
Drains with beds of rocks and gravels have a higher capacity to carry water because the rocks and gravels create a rough surface that helps to slow down the flow of water. This prevents the water from eroding the soil and causing the drain to collapse.
Drains lined with stones also have a higher capacity to carry water because the stones create a smooth surface that helps to prevent the water from eroding the soil. Additionally, the stones help to protect the drain from damage caused by roots and other debris.
Both types of drains are less likely to clog than other types of drains because the rocks and gravels or stones help to filter out debris. This prevents the debris from building up and causing the drain to become blocked.
Therefore, the maximum permissible velocity of 1.5 m/sec is adopted in drains with beds of rocks and gravels, as well as in drains lined with stones.