To determine the correct combination of roles and individuals:
1. G.V. Mavalankar was the First Speaker of the Lok Sabha.
2. B.R. Ambedkar was the Chairman of the Drafting Committee.
3. Rajendra Prasad was the President of the Constituent Assembly.
4. The role of the first leader of the opposition in Lok Sabha is not associated with Dr. Sachindananda Sinha.
Therefore, the correct answer is (2) G.V. Mavalankar : First Speaker of the Lok Sabha.