The answer is C. Population increased, roads and schools were built, cities flourished.
The reason for this is because when people heard about the gold in California, they moved over there to try and get some gold, too. More people came, and more people got gold, meaning more things to pay people for.
So the government paid people to make roads, schools, and other buildings to make California look amazing, so more people would want to come and stay there.
All in all, gold was in California, so more people came, and more things were improved.