You are stopped at a red light on a one-way street, waiting to turn left onto another one-way street, and you notice a sign beside the signal showing a left-turn arrow, a red signal lens and a red interdictory ring. What does it change?

Correct answer: A. The left turn on red that would normally be allowed here is prohibited — wait for the green before turning

It takes away a turn you would otherwise have been allowed to make. Ontario permits a left turn on red from a one-way road onto a one-way road after a complete stop, and a right turn on red after a complete stop, unless a sign says otherwise. Book 5 describes the NO LEFT TURN ON RED sign as one that "prohibits a left turn, that without the sign would be legal, during the red phase of the traffic signal", and the NO RIGHT TURN ON RED sign the same way for right turns. So the sign does not create a general ban on turning there — it means you must wait for the green, then turn. These signs are posted where drivers have been observed not stopping first, where turns get their own signal phase, or where a lot of children, older or disabled pedestrians cross.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 16 — NO RIGHT TURN ON RED Sign (Rb-79R) and NO LEFT TURN ON RED Sign (Rb-79L)