Nanda Devi National Park was declared World Heritage Site by UNESCO in the year ____.

The correct answer is (a). Nanda Devi National Park was declared a World Heritage Site by UNESCO in 1986. It is located in the Garhwal Himalayas in the Indian state of Uttarakhand. The park is home to a variety of plant and animal life, including the endangered snow leopard. It is also a popular destination for trekking and mountaineering.

Option (b) is incorrect because Nanda Devi National Park was not declared a World Heritage Site in 1988.

Option (c) is incorrect because Nanda Devi National Park was not declared a World Heritage Site in 1990.

Option (d) is incorrect because Nanda Devi National Park was not declared a World Heritage Site in 1992.