The correct answer is: B. Kangra
Kangra district has the highest road length in the state of Himachal Pradesh. It has a total road length of 3,378 km, which is more than the other three districts mentioned in the question.
Shimla district has a total road length of 2,982 km, Mandi district has a total road length of 2,671 km, and Hamirpur district has a total road length of 2,463 km.
The road length in Kangra district is mainly due to its geographical location. It is the largest district in Himachal Pradesh and is located in the central part of the state. It is also a major tourist destination, which has led to the development of infrastructure, including roads.
The road length in the other three districts is also significant. Shimla district is the capital of Himachal Pradesh and is a major tourist destination. Mandi district is also a major tourist destination and is home to several historical and religious sites. Hamirpur district is located in the southern part of the state and is a major agricultural and industrial hub.