The correct answer is C. Narrow Gauge.
Narrow gauge railways are railways with a track gauge narrower than the standard gauge of 1,435 mm (4 ft 8 1â2 in). Narrow gauge railways are often used in developing thinly populated areas because they are less expensive to build and maintain than standard gauge railways. They are also more flexible and can be built on more difficult terrain.
Broad gauge railways are railways with a track gauge wider than the standard gauge of 1,435 mm (4 ft 8 1â2 in). Broad gauge railways are often used in developed areas with high traffic volumes because they can carry more passengers and freight than standard gauge railways. They are also more stable and can handle higher speeds.
Meter gauge railways are railways with a track gauge of 1,000 mm (3 ft 3 3â8 in). Meter gauge railways are a compromise between narrow gauge and broad gauge railways. They are often used in areas with moderate traffic volumes and where there is a need for a flexible railway that can be built on difficult terrain.
Any of the above can be used in developing thinly populated areas, but narrow gauge is the most common choice.