Right answer is (c) Silt Carrying Capacity
Best explanation: The main cause of retrogression is the silt carrying capacity of the channel. This is explained as when a weir is constructed the upstream side losses it silt carrying capacity due to the formation of shoals and islands. So, therefore in order to fulfil the increased demand of the silt carrying capacity of the channel in downstream side progressive erosion takes place on downstream side leading to retrogression.