What does an artist often measure to correct proportion?

Correct answer: Length and width of parts

To correct proportion, an artist compares measurable relationships among the parts of a drawing. Measuring or estimating the length and width of features helps determine whether one part is too large, too small, too long, or too narrow in relation to another. Colour brightness, paper price, and the age of an artwork do not establish the size relationship between its drawn parts, so option D is correct.