Explanation:

- The Census of India 2001 collected slum population data across various states.

- Madhya Pradesh (Option 1): Has significant urban areas but not the highest slum population.

- Gujarat (Option 2): Also urbanized, but not at the top for slum population.

- Rajasthan (Option 3): Has lower urban density and slum concentrations compared to some other states.

- Maharashtra (Option 4): This state, especially due to Mumbai, registered the largest slum population, with a major chunk of urban poor living in informal settlements.

- Hence, Maharashtra had the largest slum population according to the 2001 Census.