What is the main effect of adding grey to blue-violet?

Correct answer: A tone of blue-violet will form and its intensity will decrease

The governing concept is colour intensity and the definition of a tone. Grey contains black and white; adding it to blue-violet generally dulls the colour and reduces its saturation, producing a tone. Therefore option A is correct. White alone would create a tint, black alone would create a shade, and grey does not increase vividness or saturation.