Introduction:
The question asks which of the following species of animals is a rare species. The given options are:
a) Wild Asiatic buffalo
b) Gangetic dolphin
c) Indian rhino
d) Black buck

Explanation:
1. Wild Asiatic buffalo:
The wild Asiatic buffalo, also known as the water buffalo or Bubalus bubalis, is a rare species. It is native to Southeast Asia and is listed as endangered by the International Union for Conservation of Nature (IUCN). Its numbers have significantly declined due to habitat loss, hunting, and competition with domestic buffalo. The conservation efforts are being made to protect this species.

2. Gangetic dolphin:
The Gangetic dolphin, also known as the South Asian river dolphin or Platanista gangetica, is an endangered species. It is found in the Ganges, Brahmaputra, and Meghna river systems in India, Nepal, and Bangladesh. The population of Gangetic dolphins has drastically decreased due to pollution, habitat degradation, and accidental entanglement in fishing nets.

3. Indian rhino:
The Indian rhino, also known as the greater one-horned rhinoceros or Rhinoceros unicornis, is listed as a vulnerable species. It is primarily found in the Indian subcontinent, particularly in Assam, West Bengal, and Nepal. The Indian rhino population has recovered from the brink of extinction, thanks to conservation efforts, but it still faces threats from habitat loss and poaching.

4. Black buck:
The black buck, also known as the Indian antelope or Antilope cervicapra, is a near threatened species. It is found in India, Nepal, and Pakistan. The black buck population has declined due to habitat loss, hunting for its meat and skin, and competition with livestock.

Conclusion:
Among the given options, the wild Asiatic buffalo is the rarest species. It is listed as endangered and faces numerous threats to its survival. The other species mentioned, the Gangetic dolphin, Indian rhino, and black buck, are also important and face conservation challenges but are relatively more abundant compared to the wild Asiatic buffalo.