Option 1: The Lava forest of Kalimpong is not known for mangrove vegetation. Mangroves are found in coastal areas with brackish water, whereas Kalimpong is inland and not close to the coast.
Option 2: The Sajnekhali forest of South 24 Parganas is indeed known for mangrove vegetation. This forest is located in the Sundarbans, which is a vast mangrove delta in West Bengal, India. The Sundarbans is a UNESCO World Heritage Site and is home to the largest population of mangrove forests in the world.
Option 3: The Dandakaranya forest of Orissa is not known for mangrove vegetation. Dandakaranya is a dense forest region in central India, predominantly consisting of tropical deciduous trees.
Option 4: Carbet National Park in U.P. (Uttar Pradesh) is not known for mangrove vegetation. It is a national park in the Caribbean island of Martinique, known for its lush rainforest and volcanic landscapes.
Therefore, the correct answer is option 2 - Sajnekhali forest of South 24 Parganas.