You leave your car standing at the roadside on a slight slope, in neutral with the engine off and the parking brake not set. It rolls forward into a fence. Beyond the damage, have you broken a rule of the road?

Correct answer: B. Yes — you must take whatever action is reasonably necessary to stop the vehicle from moving

Yes. HTA s.170(9) makes it an offence in itself: "No person shall park or stand a vehicle on a highway unless he or she has taken the action that may be reasonably necessary in the circumstances to prevent the vehicle from moving or being set in motion." Notice how the duty is phrased — not a fixed checklist, but whatever is "reasonably necessary in the circumstances." On a flat street with an automatic transmission, selecting park may be enough; on a slope it means the parking brake as well, and the Driver's Handbook adds turning the wheels toward or away from the curb. The steeper the ground, the more the law expects of you. It also applies to standing, not just parking, so a brief stop is not exempt.

Source: MTO Driver's Handbook: HTA Section 170(9) — Precaution against vehicle being set in motion