The basis for constituting the Constituent Assembly of India was the Cabinet Mission Plan.

Option 1, The Resolution of the Indian National Congress, is not the correct answer because while the Indian National Congress played a significant role in the independence movement and the formation of the Constituent Assembly, it was not the sole basis for its constitution.

Option 3, The Indian Independence Act, is also not the correct answer because although this act granted India independence, it did not provide the framework for the formation of the Constituent Assembly.

Option 4, The resolutions of the Provincial/State Legislatures of the Dominion of India, is not the correct answer either. While the resolutions of the Provincial/State Legislatures had some influence on the formation of the Constituent Assembly, they were not the primary basis.

Therefore, the correct answer is option 2, The Cabinet Mission Plan, which laid out the proposal for the formation of the Constituent Assembly. It provided the framework for the Assembly`s composition and functions.