Final answer:
Mithamo's screaming for the red tricycle in the toy store is driven by the Id, which operates on the Pleasure Principle and seeks immediate gratification.
Step-by-step explanation:
According to Freud's psychoanalytic theory, the part of Mithamo's personality most responsible for his behavior in the toy store is the Id. The Id is driven by the Pleasure Principle, which seeks immediate gratification of desires and needs. In this scenario, Mithamo's Id is dominating his actions, as he wants the red tricycle and expresses his desire through screaming without regard for the social appropriateness of his behavior or the consequences.