The All India Depressed Classes Leaders` Conference, held in Bombay in 1931, focused on the welfare of the "untouchables" in the society. The correct answer is option 2 - Separate electorates for untouchables. This was because the conference wanted to ensure the untouchables had a political voice and could fight for their rights effectively, which they believed could be ensured by having separate electorates.

Option 1, Universal adult suffrage, although a democratic ideal, was not the primary demand of this particular conference.

Option 3, Reserved seats for the minorities, although significant, does not accurately represent the unique concerns of untouchables which the conference was advocating for.

Option 4, A unitary State in India, relates to the broader issue of India`s political structure and did not specifically address the concerns of untouchables, hence making it incorrect.

To sum up, option 2 is the correct answer as it best encapsulates the central demand of the conference.