The question is about a historical event related to India`s independence, referring to proposals made in May for a united India that would form a Federal Union composed of British provinces.

Option 1 refers to the Simon Commission, which was set up by the British in 1927 to review the functioning of the constitutional system in India. But it did not pertain to a unified federal structure for India.

Option 2, the Gandhi-Irwin Pact, was a political agreement signed in 1931 between Mahatma Gandhi and the then Viceroy of India, Lord Irwin. But this agreement was about the cessation of civil disobedience movement by the National Congress against the British.

Option 3 refers to the Cripps Mission. This mission was sent in 1942 by the British government to negotiate with the Indian National Congress during the World War II, but it proposed dominion status and not a united India.

Option 4 which is the Cabinet Mission, matches the information in the question. It was sent in 1946 by the British Government to discuss the transfer of powers from the British Raj to the Indian leaders, proposing a united India. Hence, it is the correct answer.