The correct answer is option 'D', Western Ghats.

Western Ghats, also known as the Sahyadri Mountains, is a mountain range that runs parallel to the western coast of India. It is recognized as one of the eight "hottest hotspots" of biological diversity in the world and is listed as a UNESCO World Heritage Site. Here is a detailed explanation of why the Western Ghats is considered a hotspot of biodiversity:

1. Geographic Location:
The Western Ghats stretches across six Indian states, namely Gujarat, Maharashtra, Goa, Karnataka, Tamil Nadu, and Kerala. Its unique position between the Arabian Sea and the Deccan Plateau creates a diverse range of ecosystems, from coastal plains to dense forests and grasslands.

2. Species Richness:
The Western Ghats is home to a staggering array of plant and animal species. It is estimated that the region has over 7,400 species of flowering plants, including about 5,000 endemic species found nowhere else in the world. Additionally, it supports around 139 mammal species, 508 bird species, 179 amphibian species, and 290 freshwater fish species.

3. Endemism:
One of the key features of the Western Ghats is its high level of endemism, which means it has a significant number of species found exclusively within its boundaries. Many plants and animals in the Western Ghats are endemic, including the lion-tailed macaque, Nilgiri tahr, Malabar giant squirrel, and numerous species of frogs, snakes, and lizards.

4. Biodiversity Hotspot:
The Western Ghats meets the criteria of a biodiversity hotspot, which is an area with a high concentration of endemic species that is also under threat from human activities. It is considered a hotspot due to its exceptional biodiversity, high degree of endemism, and the presence of numerous threatened species.

5. Ecological Importance:
The Western Ghats plays a crucial role in regulating the climate of the region and providing various ecosystem services. It acts as a barrier to the southwest monsoon winds, leading to heavy rainfall along its windward side and creating several rivers and waterfalls. These rivers support agriculture, hydropower generation, and freshwater supply to millions of people.

In conclusion, the Western Ghats in India is a hotspot of biodiversity due to its geographic location, species richness, high endemism, and ecological importance. It is a treasure trove of unique plants and animals and plays a vital role in maintaining the overall ecological balance of the region.