The correct answer is option 4, Narmada. An estuary is a partially enclosed coastal body of water where freshwater from rivers or streams meets and mixes with seawater. It is typically formed when a river flows into an ocean or sea, and the mixing of freshwater and saltwater creates a unique ecosystem. While all rivers eventually flow into the ocean or sea, not all create an estuary. In this case, the Narmada River creates an estuary where it meets the Arabian Sea in the Gulf of Khambhat. The Krishna, Mahanadi, and Godavari rivers do not create estuaries as they directly flow into the Bay of Bengal without significant mixing with seawater.