The correct answer is: b) High construction costs due to mountainous terrain.
Sikkim is a mountainous state in the eastern Himalayas. The terrain is very rugged and mountainous, which makes it very difficult and expensive to build railways. The cost of building a railway in Sikkim would be much higher than the cost of building a railway in a flatter area.
The other options are not as significant challenges. Lack of demand is not a major issue, as there is a growing population in Sikkim and a need for better transportation. Limited government investment is also not a major issue, as the Indian government has been investing in infrastructure in Sikkim in recent years. Environmental concerns are a potential issue, but they
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