The correct option is Marshes of the Gangetic Plain.
Explanation
Wetlands in India are classified based on their geographical location and water salinity. They are broadly categorized into inland wetlands (which can be freshwater or saline depending on the drainage and climate) and coastal wetlands (which are typically saline or brackish due to tidal influence).
- Marshes of the Gangetic Plain is Correct: The marshes of the Gangetic Plain are located in the extensive floodplains of the Ganga river system. These wetlands are fed by river water and monsoon precipitation, maintaining freshwater conditions throughout the year. They represent classic inland freshwater marshes.
- Lagoons of the southern west coast is Incorrect: The lagoons of the southern west coast, such as the backwaters of Kerala (Kayals), are coastal bodies of water connected to the sea. Due to the influx of seawater, these environments are characterized by brackish or saline water.
- Mangroves of Andaman is Incorrect: Mangroves are specialized littoral forests found in intertidal zones. The mangroves of the Andaman and Nicobar Islands are adapted to survive in high-salinity environments and are not freshwater ecosystems.
- Saline expanses of Rajasthan is Incorrect: The wetlands in the arid regions of Rajasthan (e.g., Sambhar Lake) are inland drainage basins. High evaporation rates and the absence of an outlet lead to the accumulation of salts, resulting in saline or hypersaline wetlands.
Key Takeaway: Freshwater marshes are predominantly found in riverine floodplains like the Gangetic Plain, whereas coastal lagoons, mangroves, and arid inland basins are distinctively saline or brackish ecosystems.