Step 1: Understanding the context of the title 'Sir'.
The British government bestowed the title of "Sir" (Knighthood) to certain prominent individuals during the colonial period as a form of recognition. Mahatma Gandhi, a prominent figure in India's struggle for independence, returned his title of "Sir" as a protest against the British Empire's colonial rule and in support of India's quest for freedom.
Step 2: Analyzing the options.
(A) Mahatma Gandhi: Correct. Gandhi returned the title of "Sir" to the British government in 1919 to protest against the Jallianwala Bagh massacre.
(B) Rabindranath Tagore: Incorrect. Tagore returned his knighthood title in 1919 after the Jallianwala Bagh massacre, but he was not the first to do so.
(C) Jawaharlal Nehru: Incorrect. Nehru did not return a title of knighthood. He was conferred with many honors but never returned them.
(D) Dr. B.R. Ambedkar: Incorrect. Ambedkar did not return the knighthood title but was a prominent figure in Indian politics.
Step 3: Conclusion.
Mahatma Gandhi was the person who returned the title of "Sir" (Knighthood) as a form of protest. Thus, the correct answer is (A).