Which tertiary colour can be formed by adding a little green to blue?

Correct answer: Blue-green

Blue and green are adjacent on the traditional colour wheel. Mixing blue with a smaller amount of green shifts the result toward green while retaining blue as the dominant hue, producing blue-green; therefore option D is correct. Yellow-green is also a tertiary colour, but it requires yellow and green. Red-violet and red-orange involve red, which is absent here.