The threat of flooding in Rampur, Himachal Pradesh, is primarily due to heavy rains in the catchment area of the Pareechu river in Tibet. The Pareechu river is a tributary of the Sutlej, and when there is significant rainfall or a natural dam breach upstream, it can lead to an overflow into the Sutlej River, causing floods in downstream areas like Rampur.
The correct answer is (B) : Pareechu