Which mixture reduces a colour's intensity while keeping it within the same hue family?

Correct answer: Add grey

In traditional colour theory, adding grey creates a tone. A tone lowers saturation or intensity while preserving the general hue family in a muted form. Adding black mainly creates a shade by lowering value, and a large amount of a complementary or different primary colour can shift the hue or neutralise it strongly. Thus, option D gives the most accurate result.