A school bus ahead has swung its stop arm out, but you cannot see its overhead red lights from where you are. Does the stop arm on its own oblige you to stop?

Correct answer: C. Yes — the stop arm is a duty in its own right, and you may not proceed until it folds back or the bus moves off

Yes. The Highway Traffic Act writes the duty twice over — once for the lights and once for the stop arm — so either one is enough on its own. Section 175(12.1) says a driver "approaching from the rear a stopped school bus that has its stop arm actuated, shall stop and shall not proceed until the bus moves or the stop arm is no longer actuated", and s.175(11.1) imposes the same duty on a driver meeting such a bus on a road without a median. Section 175(12.2) sets the distance: a driver approaching from the rear "shall stop at least 20 metres before reaching the bus" whether it is the lights or the stop arm that is showing. So you never have to decide which signal counts — an extended stop arm is a complete instruction by itself, and it stays binding until the arm folds back or the bus moves off.

Source: MTO Driver's Handbook: Highway Traffic Act, R.S.O. 1990, c. H.8, Section 175(11.1), (12.1) and (12.2) — duty of drivers when a school bus stop arm is actuated