The correct answer is: A. river bed upstream of head regulator.
Silt excluders are constructed on the river bed upstream of head regulator to prevent silt from entering the canal. Silt is a fine-grained sediment that can be carried by water. It can cause problems in canals, such as clogging the canal and reducing the amount of water that can flow through it. Silt excluders are designed to trap silt before it can enter the canal. They are typically made of a mesh or screen that allows water to flow through but traps silt.
Option B is incorrect because silt excluders are not constructed on the river bed downstream of head regulator. This is because silt has already entered the canal by this point.
Option C is incorrect because silt excluders are not constructed on the canal bed upstream of head regulator. This is because the canal bed is already protected by the canal itself.
Option D is incorrect because silt excluders are not constructed on the canal bed downstream of head regulator. This is because silt has already entered the canal by this point.