Explanation:

India is a country with immense diversity in its physical features, climate, and biodiversity. To effectively study, manage, and conserve this rich natural heritage, the country has been systematically divided into distinct biogeographical regions. Based on the classification by the Wildlife Institute of India (WII), India is divided into $10$ major biogeographical regions. \nOption A, '$10$', is the correct answer. These $10$ regions are: Trans-Himalayan, Himalayan, Desert, Semi-Arid, Western Ghats, Deccan Peninsula, Gangetic Plain, Coasts, North-East, and Islands. Each region possesses unique ecological characteristics, flora, and fauna. \nOption B, '$15$', is incorrect. While India does have $15$ agro-climatic zones, this classification is based on agricultural parameters and is different from biogeographical regions. \nOption C, '$6$', is incorrect. This number does not correspond to any standard biogeographical or ecological classification of India. \nOption D, '$20$', is also incorrect. This number is not recognized as a standard division for India's biogeographical regions.