The Salt March occured to protest an unfair British law. Option A is correct.
The Salt March occured from March to April 1930 in India. It constituted an act of civil disobedience led by Mohandas Gandhi in an attempt to protest British rule in India.
During the Salt March, thousands of Indians followed Gandhi from his religious retreat near Ahmedabad to the Arabian Sea coast. The March Salt led to the arrest of roughly 60,000 people, including Gandhi himself. India finally became independent in 1947.