Final answer:
Rationalization is the mature defense mechanism among the provided options, as it allows a person to justify behaviors or feelings to reduce stress.
Step-by-step explanation:
Out of the options provided—regression, rationalization, projection, and displacement—the one that represents a mature defense mechanism according to psychoanalytic theory is rationalization. Mature defense mechanisms are adaptive and can evolve over time, allowing a person to optimize functioning, maintain self-image, and reduce stress. Rationalization involves an individual explaining or justifying their behaviors or feelings in a seemingly logical or rational way to avoid confronting the true underlying emotions or motivations. It is considered a more mature mechanism compared to others such as regression or projection, which often distort reality more severely.