Over one lane on an intersection approach hangs a sign showing left, right and straight-ahead arrows together — ALL MOVEMENTS PERMITTED. If everything is permitted, why post a sign at all?

Correct answer: A. It grants movements the unsigned rules would not allow from that lane — normally you may turn left only from the leftmost lane and right only from the rightmost

Because without it, everything would not be permitted from that lane. Book 5 says lane designation signs "must be used on the approaches to intersections where the permitted movement in one or more of the lanes on a given approach is contrary to the normal unsigned rules of the road", covering "permission for a normally prohibited movement, prohibition of a normally permitted movement, or both." The normal unsigned rules it lists are that straight-through movements are allowed in all continuous lanes, that "left-turn movements are usually permitted from the left-most lane only", and that "right-turn movements are usually permitted from the right-most lane only." So an ALL MOVEMENTS PERMITTED sign over a lane that is not the leftmost or rightmost is granting you turns you would otherwise not be allowed to make from there.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 11 — Lane Designation Signs, ALL MOVEMENTS PERMITTED Sign (Rb-46)