What can be one reason for getting a muddy green?

Correct answer: A trace of red paint remaining on the brush

The governing concept is contamination in colour mixing. Yellow and blue normally produce green, but a trace of red left on the brush introduces a third pigment. Because red is complementary to green, it reduces the mixture’s brightness and can make the result look greyish or muddy. Pure paints, a clean palette, and careful mixing help prevent this problem, so option B is the valid cause.