The correct answer is: c) Hilly and mountainous geography.
Nagaland is a state in Northeast India. It is one of the seven sister states and is the 12th largest state in India. The state is hilly and mountainous, with an average elevation of 1,600 meters (5,200 feet). The terrain is rugged and difficult to traverse, which makes it difficult to build railways.
The other options are incorrect. Option a) is incorrect because Nagaland is not densely populated. The population density of Nagaland is 140 people per square kilometer, which is much lower than the national average of 464 people per square kilometer. Option b) is incorrect because Nagaland does not have mostly plain terrain. The state is hilly and mountainous, with an average elevation of 1,600 meters (5,200 feet). Option d) is incorrect because Nagaland does not have major rivers. The state has a few small rivers, but they are not major rivers.