Which method is suitable for creating a dark red such as maroon?

Correct answer: Add a little blue or black to red

The governing concept is shading: adding a darkening colour lowers the lightness of a base colour. A small quantity of black deepens red, while a little blue can shift it toward the cool, deep red family associated with maroon. Hence option B is the best answer. White would create pink, extra yellow would make a warmer orange-red, and green with white would reduce the red intensity and lighten it.