Final answer:
The prohibition of slavery in California in 1849 was not a key factor contributing to its population growth; rather, the California Gold Rush and land booms were the major drivers.
Step-by-step explanation:
The factor that was not a key contributor to the dramatic population growth in California is D. the prohibition of slavery in California in 1849. California's population surged due to several events, notably the California Gold Rush and the California land boom in the 1880s. Following the discovery of gold in 1848, the so-called "forty-niners" migrated to California, significantly increasing the population, which led to California's admission as a free state as part of the Compromise of 1850. While the prohibition of slavery was part of the conditions for California's statehood, it was not directly related to the state's population growth, which was largely fueled by economic opportunities such as the gold rush and land development.