B. Keibul Lamjao National Park
Keibul Lamjao National Park is unique in being a swamp with floating vegetation that supports a rich biodiversity. Here is an explanation of why this option is the correct answer:

1. Location and Description:
Keibul Lamjao National Park is located in the northeastern state of Manipur, India. It is situated in the Loktak Lake, which is the largest freshwater lake in the country. The park covers an area of about 40 square kilometers and is characterized by its unique floating vegetation.

2. Floating Vegetation:
The main attraction of Keibul Lamjao National Park is the phumdis, which are floating islands made up of decomposed plant material, soil, and organic matter. These phumdis are formed due to the accumulation of biomass in the lake. They float on the surface of the water, creating a unique and dynamic ecosystem.

3. Biodiversity:
The floating vegetation of Keibul Lamjao National Park supports a diverse range of flora and fauna. The park is particularly known for being the last natural refuge of the endangered Manipur brow-antlered deer, also known as the Sangai. This deer species is endemic to the region and is found only in this national park.

4. Avifauna:
The park is also a haven for bird enthusiasts as it supports a rich variety of avifauna. Several species of waterbirds, migratory birds, and resident birds can be found in the park. Some of the commonly spotted bird species include the Indian cormorant, spot-billed pelican, black kite, and common moorhen.

5. Other Wildlife:
Apart from the Manipur brow-antlered deer and avifauna, Keibul Lamjao National Park is also home to various other wildlife species. This includes the hog deer, wild boar, Asiatic golden cat, leopard cat, and several species of snakes and amphibians. The park's diverse ecosystem provides a suitable habitat for these animals.

6. Importance and Conservation:
The unique floating vegetation and rich biodiversity of Keibul Lamjao National Park make it ecologically significant. The park has been recognized as a Ramsar site, which is a wetland of international importance. Efforts are being made to conserve and protect the fragile ecosystem of the park, including measures to control the spread of invasive plant species.

In conclusion, Keibul Lamjao National Park stands out among the given options as a swamp with floating vegetation that supports a rich biodiversity. Its unique ecosystem, including the presence of phumdis and the endangered Manipur brow-antlered deer, makes it a remarkable national park.