What is generally formed by adding grey to a tertiary colour?

Correct answer: Tone

The governing concept is the distinction among tone, tint, and shade. Grey is added to a colour to soften or reduce its intensity, producing a tone. This does not create a tint, which requires white, or a shade, which requires black. Nor does the mixture become a primary colour, because adding grey changes appearance rather than changing the colour category. Thus, option A is the correct answer.