A yellow sign warns CENTRE LINE MARKING ENDS. Does that mean the road is about to become one-way, or something else?

Correct answer: A. The painted centre line ends, but two-way traffic continues — often where the pavement gives way to a different surface

Something else — the line disappears but the two-way traffic does not. Book 6 says the sign "is used to alert drivers to the downstream termination of the permanent marking along the centreline of the road, designating lanes for opposing traffic", a condition that "typically occurs at locations where the pavement ends and painted markings are not possible." It spells out the three things the driver has to do about it: "drivers need to be informed that the centre line marking is ending in order to adjust their orientation within the road limits, to be aware that two-way traffic operation is continuing, and to compensate for a different road surface material." So you are about to lose the line that has been telling you where your half of the road is, on a surface that may also be loose, while traffic keeps coming the other way.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning Signs (July 2001), Section 12 — CENTRE LINE MARKING ENDS Sign (Wc-17)