Mangrove plants are found in :

Concept: Mangroves are salt-tolerant trees and shrubs that grow in the intertidal zone of tropical and subtropical coastlines — the muddy, waterlogged belt where fresh river water mixes with saline seawater during high and low tides. Their specialised roots (e.g. pneumatophores) are adapted specifically to this tidal, brackish-water, deltaic environment.

Application: The Sundarban lies across the Ganges-Brahmaputra-Meghna delta on the Bay of Bengal coast (West Bengal, India, and Bangladesh) — a low-lying, tide-affected deltaic wetland that matches this habitat exactly, and it is the world's largest contiguous mangrove forest. So among the given options, mangrove plants are found in the Sundarban.

Cross-check: None of the other options offer this tidal, saline setting:

• Himalaya is a high-altitude, cold mountain terrain with no tidal or saline water, so mangroves cannot establish there.

• Desert receives very low rainfall and has no standing brackish water, the opposite of the waterlogged conditions mangroves need.

• Rainforest, as a generic category, denotes any dense tropical forest defined mainly by high rainfall; it does not specifically denote the tidal, saline, deltaic habitat that mangroves need, so it is not the precise match here.