Explanation:

- Kunwar Singh: A significant leader in the revolt of 1857; he was a landlord in Bihar and fought fiercely against the British forces.

- Begum Hazrat Mahal: Played a key role in the uprising; as the wife of Nawab Wajid Ali Shah of Awadh, she led the rebellion in Lucknow.

- Mangal Pandey: Known for his role as a catalyst in the revolt; his actions against British officers are considered a key event that sparked the uprising.

- Bhagat Singh: A prominent freedom fighter, but not associated with the 1857 revolt; he was active during the 1920s and 1930s against British colonial rule.

Correct Answer: Option 4: Bhagat Singh