The correct answer is option 4 - The prophecy that the end of British rule would lead to the end of the Kali Yuga and the return of Ram Rajya was NOT a cause of the Revolt of 1857.

Option 1 states that the rumor that the British had mixed the bone dust of cows and pigs into the flour being sold in the market was a cause of the revolt. This refers to the controversy surrounding the use of animal fat in cartridges used by Indian soldiers, which offended both Hindu and Muslim beliefs.

Option 2 states that the prophecy that British rule would come to an end on the centenary of the Battle of Plassey on 23rd June, 1857, was a cause of the revolt. This shows the belief and anticipation among some Indians that the revolt could be a turning point in their struggle against British rule.

Option 3 states that popular discontent with British rule was a cause of the revolt. This refers to the widespread dissatisfaction among various sections of Indian society, including soldiers, peasants, and elites, due to issues such as economic exploitation, land policies, and cultural suppression.

Option 4, the correct answer, states that the prophecy that the end of British rule would lead to the end of the Kali Yuga