Final answer:
The era characterized by the rise of big business, increased unionization, extensive railroad growth, and industrialization is known as the Gilded Age.The correct option is a.
Step-by-step explanation:
The major era in US history that is described by the rise of big business, increased unionization, extensive railroad growth, and industrialization is known as the Gilded Age. This period saw rapid economic growth, the transformation from an agrarian to an industrial economy, and increased urbanization due to the expansion of the railroads and telegraph lines. During the Gilded Age, the economic landscape of the United States underwent a profound change, setting the stage for future regulatory frameworks. Issues such as corporate dominance, unionization, and government corruption gave this era a dual nature, which Mark Twain satirically referred to as 'gilded' - shiny on the outside but with underlying issues.