The correct answer is: b) Difficult geographical terrain.
Arunachal Pradesh is a landlocked state in north-eastern India. It is the largest state in the country by area, but it has a very low population density. The state is also characterized by its difficult geographical terrain, which includes mountains, forests, and rivers. This makes it difficult and expensive to build and maintain roads and other infrastructure.
The other options are also challenges faced in developing and maintaining the transport network in Arunachal Pradesh, but they are not as significant as the difficult geographical terrain. Low traffic volume is a challenge because it makes it difficult to justify the investment in infrastructure. Lack of skilled workforce is a challenge because it makes it difficult to find people with the skills to build and maintain infrastructure. Limited funding is a challenge because it makes it difficult to pay for the construction and maintenance of infrastructure.
However, the difficult geographical terrain is the most significant challenge because it makes it difficult to build and maintain infrastructure in the first place. The other challenges are also important, but they are not as difficult to overcome as the difficult geographical terrain.