You are driving along a stretch of curb signed NO STOPPING when the car ahead of you brakes for a red light and you have to stop behind it. Have you just broken the restriction?

Correct answer: B. No — stopping to avoid conflict with other traffic, or in obedience to a signal, sign or police officer, is excepted

No. Book 5 defines stopping for this purpose as "a driver stopping a vehicle in the curb lane for any reason, except when legally required to do so, for example at a STOP sign", and describes the effect of the sign this way: "Where there is a 'No Stopping' regulation in effect, motorists are not permitted to stop for any reason except to avoid conflict with other traffic or in compliance with the directions of a police officer or of a traffic control sign or signal." Sitting still because the traffic ahead has stopped, or because a red light or an officer has told you to, is exactly what those exceptions cover. What the sign forbids is stopping by choice — waiting, dropping someone off, or pulling over for a moment.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 13 — Parking Control Signs, definition of stopping; NO STOPPING Sign (Rb-55)