Option 1, Mangal Pandey, was a soldier in the British Indian Army who played a significant role in the events leading up to the 1857 revolt. He is considered a key figure in the early stages of the rebellion, but he was not believed to be a leader of the Sanyasis and Fakirs conspiring against the British.

Option 2, Bahadur Shah II, was the last Mughal emperor and a symbolic leader of the 1857 rebellion. However, he was not specifically associated with being a leader of the Sanyasis and Fakirs conspiring against the British.

Option 3, Queen Zeenat Mahal, was the wife of Bahadur Shah II but was not a leader of the Sanyasis and Fakirs. She played a role in supporting and encouraging the rebellion but was not considered a leader of the conspirators.

The correct answer is option 4, Nana Sahib. He was a prominent Indian leader who led the uprising in Kanpur and was actively involved in organizing the rebellion. He was believed to be a leader among the Sanyasis and Fakirs who conspired against the British in 1857.