A deer crossing sign carries a small yellow tab beneath it reading NIGHT DANGER. What has the road authority added by putting it there?

Correct answer: D. That the animal-crossing hazard on this stretch is particularly severe at night

A statement about when the risk peaks, not a new instruction. Book 6 says the NIGHT DANGER tab "is used in conjunction with the DEER CROSSING sign (Wc-11, Wc-111) or the MOOSE CROSSING sign (Wc-12, Wc-112) to indicate that the deer or moose crossing hazard is particularly severe at night." That is worth taking seriously on the stretch it marks: deer and moose move most at dusk and after dark, a moose is tall enough that its body sits above your headlight beam while its legs are in shadow, and the eyeshine you are scanning for only works if you are looking far enough ahead. The tab does not change what the deer sign requires — it tells you which hours on this road are the dangerous ones.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 6 — Warning Signs (July 2001), Section 8 — NIGHT DANGER Tab Sign (Wc-12t)