The correct answer is D. To control the silting.
Bed bars are artificial structures that are built in the bed of a canal to control the silting. They are made of concrete, stone, or other materials and are designed to slow down the flow of water and trap sediment. This helps to prevent the canal from becoming clogged with silt, which can reduce its capacity and efficiency.
Option A is incorrect because bed bars are not used to watch the general behavior of a canal. This is typically done by monitoring the water level, flow rate, and other parameters.
Option B is incorrect because bed bars are not used to measure the discharge of a canal. This is typically done by using a flow meter or other device.
Option C is incorrect because bed bars are not used to raise the supply level of a canal. This is typically done by using a pump or other device.