Final answer:
The ego is the part of a person's personality structure that is least stable and heavily influenced by the situation. It acts as a mediator between the id and the superego, seeking to balance their conflicting desires.
Step-by-step explanation:
The part of a person's personality structure that is least stable and heavily influenced by the situation is the ego. According to Freud's theory of personality, the ego acts as a mediator between the impulsive desires of the id and the socially acceptable standards set by the superego. The ego's job is to find a balance between these conflicting forces to satisfy the id's desires in a rational way without causing guilt or anxiety.