Ontario's warning signs are organized into lettered series (Wa, Wb, Wc, and so on) in the traffic manual that governs them. What does this series letter tell an engineer choosing a sign?

Correct answer: C. It categorizes the type of hazard the sign covers, for selecting the right sign

The Wa/Wb/Wc series groups warning signs by the type of hazard they cover (for example, alignment signs versus intersection signs versus surface-condition signs) — it's a classification code for selecting and cataloguing the right sign, not something a driver reads off the sign itself.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning sign classification (Wa/Wb/Wc series)