Step 1: Understanding light and dark in visual art
In the field of art and design, "lightness" and "darkness" refer to the variation in the value of a colour or shade.
This is a fundamental concept for creating depth, contrast, and form in a visual composition.
Step 2: Definition of 'Tone'
Tone refers to the relative lightness or darkness of a colour or a surface.
Artists use tone to create shading, volume, and a sense of three-dimensionality.
Step 3: Evaluating other options
(B) Texture $\Rightarrow$ Refers to the surface quality (rough, smooth, etc.), not light-dark value.
(C) Line $\Rightarrow$ Refers to the outline or stroke; unrelated to brightness or darkness.
(D) Form $\Rightarrow$ Refers to shape and volume, not tonal value.
Hence, the correct answer is Tone.
\[
\boxed{\text{Correct Answer: (A) Tone}}
\]