The most accurate inference based on the population data is:
The mass migration of forty-niners transformed San Francisco into an economic hub practically overnight.
This is supported by the dramatic increase in population from April 1848, when there were only 850 residents, to December 1849, when the population skyrocketed to 25,000. This rapid growth indicates that the Gold Rush had a significant and immediate impact on the city, establishing it as a major destination for those seeking fortune and contributing to its transformation into an economic center.