The correct answer is B) be both audible and visual. The high-level overfill tank alarm is required to have both audible and visual indicators to alert the operator when the tank is nearing full capacity. This is to ensure the operator is clearly aware of the tank's status and can take appropriate action to prevent an overfill situation. The alarm must be distinct from the actual overfill alarm, which is a separate safety system. The other options are incorrect because A) the alarm does not need to sound at a specific 90% tank level, C) the alarm does not need to operate in unison with other alarms, and D) the high-level alarm is a separate system from the overfill alarm.