Hi there!
The answer you are looking for is option A. Pattern and rhythm.
Pattern and Rhythm are principles of Desing.
A pattern is a repetition of design, an element that occurs over and over again in a composition. In this case, there are two examples of repetitions. The first one is the repeated use of alternating same size red and white stripes. The second one is the alternating rows of same size white stars on the blue background.
On the other hand, rhythm is the repetition of shapes, lines and forms to produce the look and feel of movement in an artwork. In the American flag, this can be seen in the repeating and set distance between the white stars on the blue background, and also through the use of same size stripes going from top to bottom of the flag alternating colours.
Hope this helps!
