Two men have been arrested in connection with the disappearance 25 years ago of the California college student Kristin Smart, the authorities said on Tuesday. Paul Flores, who was the last person to see Ms. Smart on the day she disappeared in May 1996 and whom the authorities had declared in recent months to be a “prime suspect” in the case, was taken into custody at his home in San Pedro, Calif., on Tuesday, Ian Parkinson, the San Luis Obispo County sheriff, said at a news conference.