What should be kept constant to obtain uniform width in poster lettering with a flat brush?

Correct answer: Brush angle and pressure

The governing concept is consistent brush contact. With a flat brush, keeping the angle and applied pressure steady keeps the same part of the brush edge against the paper, so the stroke remains approximately uniform in width. Changing the angle alters the contact area, and changing pressure can spread or narrow the stroke. Water affects flow and drying, but does not alone guarantee width; letter spacing and paper orientation are unrelated. Therefore, option A is correct.