Correct Answer: Construction of border roads in high-altitude areas.

Solution : The correct answer is Construction of border roads in high-altitude areas.

National Highways in India are primarily meant for inter-state transport; they connect state capitals and major cities and are maintained by the Central Government. However, the construction of border roads in high-altitude areas, especially in remote and mountainous regions, is typically undertaken by specialized organizations like the Border Roads Organization (BRO) rather than being a primary function of National Highways.