Final answer:
The San Francisco Vigilance Committee is best described as having preserved law and order through extrajudicial means in response to local governmental failures in the 1850s. Option B is correct.
Step-by-step explanation:
The phrase that BEST describes the duties of the San Francisco Vigilance Committee is Preserving law and order. The committee was formed by private citizens in 1851 and again in 1856 due to widespread crime and corruption in law enforcement. Its role was to take on extrajudicial actions to ensure that justice was served when the official law enforcement failed to do so. They operated independently and were seen as a civic response to ineffective policing and local government.