A nation's buildings, roads, bridges, power grids, etc.

A. Infrastructure.

Explanation:

The buildings, roads, bridges and so on are part of a country's infrastructure, the set of (public or not) facilities that allow for the function of its living conditions and economy. For example, bridges are built so transportation may be made through ways that were previously unavailable for any or most kind of transport to go through, being made by a company (state's or private) with the intention of providing better access from and to different areas.