The correct answer is (a). Arunachal Pradesh is considered a biodiversity hotspot due to its rich variety of flora and fauna. It is home to over 300 species of birds, 100 species of mammals, and 1,000 species of plants. The state is also home to a number of endangered species, such as the snow leopard, the red panda, and the clouded leopard.
(b) is incorrect because high altitude does not necessarily lead to a high level of biodiversity. For example, the Himalayas are a high-altitude mountain range that is home to a relatively low number of species.
(c) is incorrect because proximity to China does not necessarily lead to a high level of biodiversity. For example, the Korean Peninsula is located near China but has a relatively low level of biodiversity.
(d) is incorrect because sparse population does not necessarily lead to a high level of biodiversity. For example, the Sahara Desert is a sparsely populated region that has a relatively low level of biodiversity.