What term refers to returning to a former or less developed state?

The term that refers to returning to a former or less developed state is "regress." This word is commonly used in various contexts, including psychology, biology, and general usage, to describe a process of moving backward or reverting to a state that was previously left behind. In contrast, the other terms provided have different meanings: "advance" typically means to move forward or make progress, "maintain" relates to keeping something in its current state without changes, and "evolve" involves developing or undergoing gradual change towards a more complex or improved form. Thus, "regress" uniquely captures the idea of moving back to an earlier condition, making it the correct choice for this question.