Final answer:
According to Kevin Starr, the completion of the transcontinental railroad was the proof that California had truly become part of the Union in the 1860s, symbolizing its economic and transport integration post-Gold Rush statehood.
Step-by-step explanation:
According to Kevin Starr, the proof positive that California had truly become part of the Union in the 1860s was A. The completion of the transcontinental railroad. This major event not only connected the West Coast with the rest of the United States physically but also symbolized California's integration into the Union's economic and transport systems. The Gold Rush sparked a massive population surge in California, ultimately leading it to become the 31st state in 1850, entering the Union as a free state. This admission was pivotal amidst the era's grave sectional conflicts over slavery and state sovereignty.
The impetus for California's statehood was the discovery of gold at Sutter's Mill in 1848, which drew thousands of settlers during the Gold Rush. This rapid population increase necessitated an organized government and propelled California directly to statehood with no territorial status interim. Post-statehood, the transcontinental railroad, completed in 1869, further cemented California's position as an integral piece of the country. It revolutionized travel and trade, marking a new era of national unity and development.
It's important to delineate the timeline accurately: While the initial influx of settlers occurred due to the Gold Rush in 1849, it was the completion of the railroad in the following decade that signified California's full integration into the Union within the context of nation-building in the 1860s.