Which one of the following districts is having the only National Park of Bihar?

The correct answer is: C. West Champaran

The Valmiki National Park is a national park in the Indian state of Bihar. It is located in the West Champaran district, about 100 km from Patna. The park was established in 1973 to protect the endangered Bengal tiger. It is home to a variety of other wildlife, including leopards, sloth bears, elephants, and deer. The park is also a popular destination for birdwatching.

The other options are incorrect because they are not districts in Bihar. Nalanda is a district in the state of Bihar, India. Bhagalpur is a city and a municipal corporation in the Indian state of Bihar. East Champaran is a district in the Indian state of Bihar.