Border Roads are constructed by the Border Roads Organisation (BRO). The BRO is a government organization under the Ministry of Defence, responsible for the construction, maintenance, and upgradation of roads in border areas of India. Let's understand in detail why the correct answer is option 'D'.

1. Introduction to the Border Roads Organisation (BRO):
- The Border Roads Organisation (BRO) was formed on 7 May 1960.
- It is responsible for the development and maintenance of road networks in India's border areas.
- The organization plays a crucial role in providing connectivity to remote areas, especially in the northern and northeastern parts of the country.

2. Role and Functions of the Border Roads Organisation:
- Construction and maintenance of roads: The primary function of the BRO is to construct and maintain roads in border areas. These roads include strategic roads, highways, and feeder roads.
- Development of infrastructure: The BRO is involved in the development of infrastructure like bridges, tunnels, and airfields, which are essential for the defense forces deployed in remote areas.
- Quick response during emergencies: The BRO is equipped to respond swiftly during natural calamities, providing immediate assistance in terms of road clearance and restoration.
- Snow clearance: The organization plays a crucial role in clearing snow from roads, especially in areas prone to heavy snowfall, ensuring connectivity even during harsh winters.
- Maintenance of border roads: The BRO is responsible for the regular maintenance and repair of border roads to ensure their operational efficiency.
- International projects: The BRO has also undertaken road construction projects in neighboring countries, contributing to regional connectivity and cooperation.

3. Importance of Border Roads:
- Strategic significance: Border roads are of utmost importance for India's defense forces, as they provide vital connectivity to remote areas near the borders. These roads facilitate the movement of troops, equipment, and supplies.
- Socio-economic development: Border roads also play a significant role in fostering socio-economic development in remote and inaccessible areas. They improve accessibility, promote trade, and enable the delivery of essential services.
- Connectivity to border areas: Border roads ensure connectivity to areas near the borders, enabling the local population to access basic amenities, healthcare facilities, education, and markets.
- Tourism and trade promotion: Well-constructed border roads enhance tourism potential and promote cross-border trade, contributing to economic growth and cross-cultural exchange.

In conclusion, the correct answer to the question is option 'D' - the Border Roads Organisation (BRO) is responsible for the construction of border roads in India. The BRO's efforts in building and maintaining these roads are crucial for national defense, socio-economic development, and connectivity to remote areas.