The correct answer is option 4, Cabinet Mission plan. The Constituent Assembly of India was a body formed to draft the Constitution of India. It was set up under the Cabinet Mission plan, which was proposed by the British government in 1946.

Option 1, Simon Commission proposals, is incorrect. The Simon Commission was a British inquiry commission that was appointed in 1927 to propose reforms for India, but it did not lead to the formation of the Constituent Assembly.

Option 2, Cripps proposals, is also incorrect. The Cripps proposals were put forward by Sir Stafford Cripps in 1942 as a plan for granting Dominion status to India, but it did not result in the formation of the Constituent Assembly.

Option 3, Mountbatten plan, is not the correct answer either. The Mountbatten plan, also known as the Indian Independence Act, was the plan that led to the partition of India in 1947, but it did not set up the Constituent Assembly.

Therefore, the correct answer is option 4, Cabinet Mission plan.