Answer:
The correct answer is d. No known cause
Explanation:
The reason why is option D is because there might be multiple factor that contributed in the end of the indus valley civilization, many specialist in the field of study believe that a strong cause is the drought of the Indo river, which overflowed and left fertile sediments so the whole civilization was dependent to the river, but there are also other known causes like for example the trade decline from Egypt and Mesopotamia as well as deforestation, flooding etc...
Therefore the correct answer to your question is D because there's not a proved cause that caused the Indus Valley civilization to disappear, it could be all of them together (according to some scientific claims) but since there's no "all of them" option the best option is D. because there is no known proved cause.
