Which principle of design does a designer apply when they create rhythmic movement in a garment by using recurring pleats or details

Repetition, and regular rhythm

Explanation:

Its certainly repetition method, and repetition method is of three types which are repetition, pattern and rhythm.

Remember repetition can be boring at times as we need to read same thing again and again. However, repetition sometimes can be good like the one we are referring to, and the other one can be like placing the logo or menu at same place in all web page. And its since it creates a habit in us for reading the web pages, And hence we can understand faster, and it looks please to our eyes hence as well.