In which situation will blue appear cooler?

Correct answer: Beside orange

The governing concept is relative colour temperature. Blue is generally cool, but its coolness appears stronger when it is placed beside a clearly warm colour. Orange is warm, so the warm-cool opposition makes the blue look especially cool and emphasizes their difference. Thus option D is correct. Green and violet can also be perceived as cool or relatively cool, and another blue is similar, so those pairings create less contrast in temperature than blue beside orange.