Question

Mention the probable causes for the decline of the Harappan
culture.

Answer 1

scientists are searching for it we can't give any specific reason for it.it may be drought,epidemics o flood.it may be also earthquake or other any disaster

Answer 2

There are no proper reasons which give us an idea for the decline of the harappan culture.
based on the speculations of historians the following causes may have led to the decline 
1) attack of Aryans - the destruction caused may have led to the decline 
2)climatic disaster
3) natural calamity like earthquake, flood