Which of the following National Parks is unique in being a swamp with floating vegetation that supports a rich biodiversity?

The correct option is Keibul Lamjao National Park.

Here is a detailed, step-by-step explanation of why this park is unique:

1. Unique Floating Ecosystem (Phumdis): Keibul Lamjao National Park, located in the Bishnupur district of Manipur, India, is the world's only floating national park. It is characterized by floating decomposing plant materials locally known as phumdis. These phumdis form a thick, spongy, and floating mat of vegetation, soil, and organic matter on Loktak Lake.

2. Support for Rich Biodiversity: This unique swampy ecosystem supports a rich and diverse variety of flora and fauna. Most notably, it serves as the last natural refuge and habitat for the critically endangered Eld's deer subspecies, the Sangai (Rucervus eldii eldii), also commonly referred to as the Manipur brow-antlered deer or the "dancing deer." The Sangai is specially adapted to run on the floating phumdi pasture.

3. Distinction from Other Options:
- Bhitarkanika National Park: Located in Odisha, it is a mangrove wetland famous for saltwater crocodiles and olive ridley turtles, not a floating swamp.
- Keoladeo Ghana National Park: Located in Rajasthan, it is a man-made wetland sanctuary renowned for migratory birds, but does not feature floating vegetation.
- Sultanpur National Park: Located in Haryana, it is a shallow freshwater lake sanctuary frequented by local and migratory birds, with no floating swamp characteristics.