In which situation will adding white lighten a colour but fail to create a more natural shadow?

Correct answer: When a deep shadow is needed

In colour theory, adding white creates a tint by increasing lightness. A deep shadow, however, requires a darker value and often a slightly cooler, warmer, or complementary adjustment depending on the subject. White would lighten the paint instead of deepening the shadow, so option D is correct. The other situations can reasonably require a lighter tint or highlight.