Since Kamala Markandaya's novel was published in 1954, the correct answer to your question would be India's independence, which occurred in 1947. Effectively, two hundred years of British rule was ended in that year, with British India divided into the nations of India and Pakistan.
Indian independence in 1947
The other three answers are not correct because:
a) Gandhi's Salt March occurred on March 12, 1930. The march was a protest against the British monopoly of salt production in India. Indians had to purchase salt at a significantly higher price from the British because they were not allowed to process salt locally. Gandhi hoped that his peaceful salt protest would focus Muslim and Hindu efforts on defying colonial rule instead of on sectarian infighting.
Gandhi's Salt March
b) The Sepoy Rebellion occurred in 1857. History tells us that the sepoys rose up against the rule of the British East Indian Company because they were angered by the British appropriation of power in local states/districts. With the growing presence of missionaries, the sepoys also became alarmed that the British may have been trying to convert all Indians into Christians.
The Sepoy Rebellion of 1857.
c) The Green Revolution in India occurred in the 1960s and the 1970s. This was a revolution in agriculture between 1967 to 1978 which propelled India to the forefront of agriculture production on the global stage.
The Green Revolution in India