Dr. Rajendra Prasad, who later became the first president of independent India, served under the portfolio of Food and Agriculture in the Interim Government formed in 1946. This government came into stance in the years prior to India gaining independence from British rule.

Option 1, Defence, is incorrect. This portfolio was held by Baldev Singh in the Interim Government of 1946.

Option 2, External Affairs and Common Wealth Relations is also not the right choice. Jawaharlal Nehru held this portfolio at the same time.

Finally, as we now know that Dr. Prasad held the portfolio of Food and Agriculture, option 4, None of these, is not the correct answer as it contradicts the factual information.

Hence, option 3, Food and Agriculture, is, in fact, the correct choice. It is the portfolio that was held by Dr. Rajendra Prasad in the Interim Government formed in 1946.