1. The correct answer should be A. use numbers to describe colors.
If you take a look at the image I've attached below, you will see that this model is used to show colors according to three color dimensions, which are hue, value, and chroma. Additionally, each of the colors is numbered for better and easier understanding.
2. Value is A. the lightness or darkness of a color.
As I've already mentioned, colors have three dimensions: hue, chroma, and value. Hue refers to the actual color of a color, its tint, what you see when you look at a color. Chroma is the saturation - how bright or dull a color is. Value is similar to that - it refers to whether a color is dark or light.
3. The correct answer is gradation.
Gradation is the term which refers to the range of colors. This range may start from lighter to darker hues of a color, or vice versa, but there is a steady increase/decrease in the value of that particular color, which is known as the gradation - they gradually become lighter or darker.