Emphasis is the principle of creating a centered interest in an artwork, often achieved by color contrast and by lines that direct the eye to it.
Using this definition, we can see all of the above. Color contrast and lines that lead the viewer's focus to a focal point.
So, it can't be chiaroscuro because there is no obscurity.
It can't be sfumato because there are very few soft tones (lots of contrast)
It can't be primary colors because we see more than just the three primaries.
That leaves complementary colors, which makes sense. The complementary colors put together creates lines that lead our attention.
Answer: b.