The correct answer is - C. the San Andreas fault.
San Francisco, as most of the western of California, is lying in close proximity to the San Andreas fault. This fault is a continental transform fault that is formed as a result of the transform boundary between the Pacific plate and the North American plate, with a small plate lying in between them. Because of the horizontal movement of the plates and the pressure they cause to one another, there's constant adjustments inside the crust, and that contributes to lot of earthquakes in the region. One of those earthquakes was the San Francisco earthquake of 1906, a very strong and devastating earthquake, often considered as the worst earthquake in the history of the United States.
