Final answer:
Winter tires or chains are necessary when a road sign indicates slippery road conditions ahead, such as during icy or snowy weather.
Step-by-step explanation:
Winter tires or chains must be used when a sign is displayed, indicating a need for increased vehicle traction. The correct situation that would require the use of winter tires or chains is b) Slippery road ahead. Highway signs indicating slippery conditions typically feature an icon of a car with skid marks, alerting drivers to potential hazards due to snow, ice, or wet conditions, and the need for winter tires or chains. Other scenarios such as school zones or construction zones do not inherently require special tire equipment but are instead related to speed limits and driver caution.