The correct option is "the discovery of gold"
California gold rush was a social phenomenon occurred in the United States between 1848 and 1855, characterized by the large number of immigrants who came to the vicinity of San Francisco (California) in search of the metal. This phenomenon began near the town of Coloma, when gold was discovered in Sutter's Mill. When the news of the discovery spread, around three hundred thousand people emigrated to California from the rest of the United States and other countries.
