What is the correct way to change red-violet into blue-violet?

Correct answer: Add blue

The governing concept is movement between neighbouring hues on the colour wheel. Red-violet contains a noticeable red influence; increasing blue shifts its balance toward blue-violet. Thus option C is correct. Adding white would produce a lighter tint without selecting the blue direction, and adding black would produce a darker shade. Yellow would move the mixture toward a duller, more neutral or contrasting result rather than the intended blue-violet.