The correct answer is option 2: Tapti. An estuary is a partially enclosed coastal body of water where freshwater from rivers meets and mixes with the saltwater of the ocean. The Tapti River forms an estuary before it empties into the Arabian Sea.

Option 1, Ganga, does not form an estuary. The Ganga River flows into the Bay of Bengal without forming an estuary.

Option 3, Godavari, and option 4, Mahanadi, do not form estuaries either. Both rivers also flow directly into the Bay of Bengal without forming estuaries.

Therefore, the only river among the options that forms an estuary is the Tapti River.

Alert - correct answer should be option 3: Godavari.