Final answer:
Charles Cora shot U.S. Marshal William Richardson due to a personal dispute over a gambling debt. Therefore, the correct answer is A. Personal dispute over a gambling debt.
Step-by-step explanation:
Charles Cora, a gambler, had a personal conflict with U.S. Marshal William Richardson over an unsettled gambling debt. This dispute escalated, leading to a confrontation where Cora fatally shot Richardson. The incident occurred in San Francisco during the 1850s, amidst a period of lawlessness and tensions in the rapidly growing city. Cora's act of shooting Richardson stemmed directly from this personal conflict rather than broader issues like racial tensions, political rivalry, or property disputes. The correct answer is A. Personal dispute over a gambling debt.