How may a red object generally appear under blue light?

Correct answer: Dull, dark, or violet-biased

The perceived colour of an object depends on both its surface colour and the light illuminating it. Under blue light, a red surface receives little of the red illumination it normally reflects, so its red appearance can become weak, dark or shifted toward a violet impression. Option D is therefore the best general answer. It need not become pure violet; the exact result depends on the light and material.