Well before a big freeway junction, an overhead green sign shows what looks like a small map — a white plan view of the road splitting, with route numbers placed against each branch. What is it for, and where will you meet it?

Correct answer: B. A diagrammatic sign, giving a plan view of a complex split at least a kilometre before the ordinary advance sign

It is a diagrammatic sign, and it comes early. Book 8 says these signs "allow road users to more easily interpret upcoming complex decision points by graphically representing the decision point on the sign", and that they "are most effective when used as a pre-advance message, located a minimum of 1 km in advance of the applicable Advance Sign." They are reserved for junctions where words alone would not do the job — left exits, two right exits too close together for separate signs, and freeway-to-freeway splits. The graphic is drawn so that "the through route is the visually dominant portion", and route numbers and destinations are placed to line up with the branch they belong to, so the sign is read by matching your route to a limb of the drawing rather than by reading top to bottom.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 8 — Guide and Information Signs, Volume 1 (May 2010), Section 5.8 — Diagrammatic Signs (G102)