Final answer:
The population of California decreased after the discovery of gold primarily due to mass emigration to other regions.
Step-by-step explanation:
The population of California decreased after the discovery of gold primarily due to B) Mass emigration to other regions.
The California Gold Rush in the mid-19th century attracted thousands of prospectors from all over the world, causing a significant decline in the population of California. People migrated to California in search of gold and economic opportunities, which led to a sudden surge in the population. However, once the gold rush began to decline and the opportunities diminished, many people left California to seek fortune elsewhere, resulting in a decrease in population.