At the intersection ahead you see a white square sign showing a straight-up arrow inside a red ring with a line through it. You had intended to carry straight on. What are your options now?

Correct answer: A. You must turn either left or right — the straight-through movement is prohibited here

Turn left or turn right — those are the only ways out. Book 5 says the NO STRAIGHT THROUGH sign "prohibits straight-through movements, and requires that the vehicle turn to either the left or right." It is not an advisory and it is not a warning that the road ends: it removes one movement from the intersection and leaves you the other two. Turn control signs like this are normally posted at the intersection itself, though an extra one may be placed in advance of it — and where an advance sign is used, there must be no other intersections, driveways or entrances in between, so an advance sign always refers to the next intersection you reach.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 8 — NO STRAIGHT THROUGH Sign (Rb-10)