The correct answer is: D. To raise water level.
A diversion head works is a structure that is used to divert water from a river into a canal. The main function of a diversion head works is to raise the water level in the canal so that it can be used for irrigation, drinking water, or other purposes.
Option A is incorrect because the main function of a diversion head works is not to remove silt. Silt is a type of sediment that is carried by water. Diversion head works may have silt traps, but these are not their main function.
Option B is incorrect because the main function of a diversion head works is not to control floods. Flood control structures are designed to prevent or reduce the damage caused by floods. Diversion head works may be used in flood control systems, but they are not the main type of flood control structure.
Option C is incorrect because the main function of a diversion head works is not to store water. Water storage structures are designed to store water for later use. Diversion head works may be used in water storage systems, but they are not the main type of water storage structure.