Final answer:
The part of the personality in Freud's psychodynamic theory that follows the morality principle is the superego.
Step-by-step explanation:
According to Freud's psychodynamic theory, the part of the personality that operates by the morality principle is the superego. The superego develops through social interactions and acts as a conscience, guiding behavior to adhere to societal norms and personal ideals. It contrasts with the id, which operates on the pleasure principle, seeking immediate gratification, and the ego, which operates on the reality principle, balancing the id and superego in the context of reality.