You are going straight through a T-intersection where a centre lane has opened up for left-turning traffic. A sign reads "THROUGH TRAFFIC KEEP RIGHT". Why is it there?

Correct answer: D. Traffic going straight should move into the right lane, leaving the left or centre lane for vehicles turning left

To keep you out of the lane the left-turners need. Book 5 says the purpose of this sign "is to direct straight through traffic into the right lane so that it is not impeded by traffic turning left", and that it "is intended to alleviate conflicts and congestion that would result if both through traffic and left-turn traffic shared the left lane." It is used on the through roadway at T-intersections where a centre lane is provided for left turns, and at other intersections where drivers going straight have been sitting in the left-turn lane. Where an existing through lane converts into a left-turn lane, the sign goes up before the left-turn arrows are painted on the pavement — so it is your warning to move over while there is still room to do it comfortably.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 5 — Regulatory Signs (December 2021), Section 10 — THROUGH TRAFFIC KEEP RIGHT Sign (Rb-27)