At a busy intersection you know has traffic lights, someone has set up a STOP sign on a portable stand facing your lane. What does that tell you about the intersection right now?

Correct answer: C. The signals are inoperative, or a traffic control person is directing traffic — a portable STOP sign is only allowed in those temporary situations

That the normal control has failed or been suspended. Book 5 keeps STOP signs and signals strictly apart: "STOP signs must not be used on the same approach to an intersection where traffic control signals are operating, as the conflicting commands of two types of control devices would be confusing." It then names the only situations in which a temporary one is allowed: "portable or part-time STOP signs must not be used except in emergency or temporary situations, such as in conjunction with Traffic Control Persons or at intersections where traffic signals are inoperative." So a portable STOP sign where lights normally hang means the lights are out or a traffic control person is running the intersection — treat it as a stop-controlled intersection and expect the other approaches to be doing the same.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 2 — STOP Signs, Purpose and Background