The correct answer, 'Prohibited sign,' refers to the common road sign which indicates that entry is not allowed. This is a red circular sign with a white horizontal bar, a universal symbol for restrictions, as per Ontario's Highway Traffic Act. In practical terms, the sign is found at areas where entry from a particular direction is forbidden, ensuring safety by preventing vehicles from entering a potentially hazardous area or a one-way street in the wrong direction. Drivers should remain vigilant for such signs to maintain traffic flow and safety.