How should black be added to preserve the brightness of yellow?

Correct answer: In a very small amount

The governing concept is controlled pigment mixing. Black is a highly dominant pigment, so even a small quantity absorbs much of the reflected light from yellow and makes it darker and duller. Therefore, black should be added gradually in a very small amount, with the mixture checked after each addition. Equal or larger amounts would overpower the yellow, while thick strokes describe an application method rather than a mixing proportion. Thus, option A is correct.