Who among the following key figures of the Indian freedom struggle was popularly known by the honorific title "Grand Old Man of India"?

The correct option is Dadabhai Naoroji.

Dadabhai Naoroji (4 September 1825 – 30 June 1917) was an eminent Indian political leader, merchant, scholar, and writer, fondly known as the "Grand Old Man of India" and the "Unofficial Ambassador of India".

Key Contributions & Facts:
1. Founding of the INC: He was one of the founding members of the Indian National Congress (INC) in 1885 and presided over its annual sessions three times (in 1886, 1893, and 1906).
2. Member of British Parliament: He was the first Indian to be elected to the British House of Commons, serving as a Liberal Party MP for Finsbury Central between 1892 and 1895.
3. Drain of Wealth Theory: He authored the seminal book Poverty and Un-British Rule in India, in which he propounded the "Drain of Wealth" theory detailing how Britain systematically siphoned wealth out of India.
4. Declaration of Swaraj: During the 1906 Calcutta session of the Congress under his presidency, the demand for Swaraj (self-rule) was formally adopted for the first time by the INC platform.