The correct answer is: A. Shimla-Pathankot
The longest state highway in Himachal Pradesh is NH 5, which connects Shimla to Pathankot. The highway is 270 kilometers long and passes through the districts of Shimla, Solan, Sirmaur, Mandi, Kullu, and Hamirpur. The highway is a major tourist route and is also important for trade and commerce.
Option B, Shimla-Solan, is a shorter highway that is 120 kilometers long. It passes through the districts of Shimla and Solan.
Option C, Shimla-Sirmaur, is a 150-kilometer-long highway that passes through the districts of Shimla and Sirmaur.
Option D, Kangra-Mandi, is a 180-kilometer-long highway that passes through the districts of Kangra and Mandi.