Why can red-violet become dull when an orange-biased red is used?

Correct answer: The orange bias introduces a yellow influence

The governing concept is unwanted complementary influence in pigment mixing. Orange contains a yellow tendency, and yellow is complementary to violet; when that influence enters a red-violet mixture, it can partially neutralise the violet and make the result duller. Thus, option A is correct. Orange bias does not increase violet, red-violet requires more than blue alone, and violet does have complementary relationships.