What is this sign warning you about?

Correct answer: C. The bridge ahead can be lifted or swung open to let watercraft through, so traffic may be stopped there

A bridge that moves. Book 6 says the purpose of the OPENING BRIDGE sign "is to alert motorists of bridges that, when in operation, can be lifted or swung to permit the passage of watercraft." That is a warning with real consequences, because when the bridge opens the road simply is not there: traffic is held by gates and, at many crossings, by a full red-amber-green traffic signal timed with them. The practical reading is that a queue at this bridge may be a long one and is not moving until a boat has cleared, so a stopped line here is not a collision or a fault — and never drive around a lowered gate at a movable bridge for the same reason you would never do it at a railway crossing.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning Signs (July 2001), Section 4 — OPENING BRIDGE Sign (Wa-30)