The correct answer is B. Nilgiri Mountain Railway.
The Nilgiri Mountain Railway is a UNESCO World Heritage Site in India. It is a mountain railway line that runs from Mettupalayam in the state of Tamil Nadu to Ooty in the state of Kerala. The railway was built in the 19th century and is one of the oldest mountain railways in the world. It is a popular tourist attraction and is known for its scenic views.
The other options are incorrect.
A. The Western Ghats are a mountain range in India. They are not a UNESCO World Heritage Site.
C. The Mahabodhi Temple is a Buddhist temple in Bodh Gaya, India. It is a UNESCO World Heritage Site.
D. The Group of Monuments at Mahabalipuram is a group of Hindu and Jain temples in Mahabalipuram, India. It is a UNESCO World Heritage Site.