Which is a simple way to make grey?

Correct answer: Mix black and white

The governing concept is mixing black and white to create a neutral grey. Black contributes darkness and white contributes lightness; changing their proportion changes the value, so more white gives a lighter grey and more black gives a darker grey. Thus option C is correct. Red plus yellow makes orange, blue plus yellow generally makes green, and red plus white makes pink or a light red tint rather than neutral grey.