What is the difference between adding black and adding yellow to violet?

Correct answer: Black darkens value while yellow reduces saturation

The governing concept is the distinction between value and saturation. Adding black lowers violet’s value and creates a darker shade. Yellow is complementary to violet; mixing a little yellow generally neutralises or dulls violet, reducing its saturation. Thus option A is correct. Neither additive lightens violet, yellow is not neutral in this relationship, and neither mixture can convert a secondary colour into a primary colour.