Above a busy downtown intersection there is a sign panel that is showing nothing at all — a dark, empty face where a turn restriction is sometimes displayed. What should you take from a blank panel?

Correct answer: A. The restriction it displays is not in effect right now — its legend appears only while the restriction applies

That the restriction is not in force at this moment. Turn control and lane designation signs "may be replaced by an electrically or mechanically operated blank-out sign bearing the legend of the corresponding sign number", and Book 5 sets the rule that makes them readable: "the legend of the electrically or mechanically operated sign must be visible only during the times that the restriction is in effect." So when the panel lights up with a NO LEFT TURN symbol, the ban is on; when it is blank, it is not. Note the difference from a traffic signal, where a completely dark signal head means the equipment has failed and the intersection must be treated as an all-way stop — with a blank-out sign, blank is a message, not a fault.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 8 — Turn Control Signs, and Section 11 — Lane Designation Signs, electrically or mechanically operated signs