A recurring sequence of thick, thin, medium, and thin lines is an example of what?

Correct answer: Four-unit complex alternating rhythm

The governing concept is visual rhythm: repetition of visual units creates an organised sense of movement. Here, the smallest complete unit contains four line qualities—thick, thin, medium, thin—and that same four-part arrangement repeats. Because the unit has several changing elements rather than a simple gradual increase or decrease, it is best described as a four-unit complex alternating rhythm. It is not random, and no continuous curve is involved, so A, C, and D are unsuitable.