The correct answer is option 4, Vembanad Lake. Vembanad Lake is the longest lake in India. It is located in the state of Kerala, and it stretches over a distance of about 96.5 kilometers. The lake is also known as Vembanad Kayal. It is a brackish lagoon, meaning it contains a mix of saltwater and freshwater. Vembanad Lake is a significant tourist attraction and is famous for its scenic beauty, backwater cruises, and various species of flora and fauna. It also plays a crucial role in the socio-economic development of the region, supporting fishing and agriculture activities. The lake is a habitat for several species of birds, including migratory birds from different parts of the world.