Which act is primarily associated with the promotion of national expansion through granting lands?

The Pacific Railway Act is indeed significant for promoting national expansion, particularly in the context of railroad construction across the United States. Enacted in the 1860s, this act facilitated the development of the transcontinental railroad by providing federal support, including land grants, to railroad companies. These grants enabled railroads to secure land needed for tracks and associated infrastructure, ultimately linking the eastern and western parts of the country and fostering economic growth, settlement, and national unity.

The other choices represent legislation with different focuses. The Homestead Act aimed to encourage settlement by offering land to individuals willing to cultivate it, but it is more associated with individual land ownership rather than the promotion of national transport and infrastructure. The Land Grant Act, while also related to land distribution, specifically refers to acts providing lands to states for educational institutions and would not be considered in the same context of national expansion via railways. The Interstate Commerce Act primarily addressed regulation of railroads and interstate commerce, rather than promoting land expansion directly.