Which quality changes first when the amount of red is steadily increased in a colour mixture?

Correct answer: The hue of the mixture

The governing concept is hue as the basic colour family of a mixture. When the proportion of red is steadily increased, the balance of pigments changes, so the mixture shifts toward a redder hue. Its exact lightness or saturation may also change depending on the other pigments, but the direct and first-described change is the colour identity or hue. Paper thickness, brush length and drawing size are physical features of the materials or artwork, not properties changed by the pigment ratio. Thus, option A is correct.