Final answer:
The most severe sanction a board can issue is license revocation, which ends the professional's ability to practice legally.
Step-by-step explanation:
The most severe sanction a board can issue to a professional holding a license is License revocation. When addressing professional conduct and adherence to standards, formal sanctions are used by licensing boards to enforce compliance with regulations. Among the sanctions listed, license revocation is considered the most severe because it completely terminates the individual's legal authority to practice their profession. License suspension temporarily removes professional privileges, but those could be reinstated. License probation imposes conditions on the professional's ability to practice, and a written reprimand is a formal disapproval that goes on record but does not prevent the professional from continuing to practice.