The textbook states that "eysenck worked diligently to unearth [the] biological basis" of the basic dimensions of personality. he appears to have been most successful in the case of:

He appears to have been most successful in the case of "extraversion".

Extraversion is one of the five personality traits of the Big Five identity theory. It demonstrates how cordial and social an individual is. An individual who scores high in extraversion on an identity test is the life of the gathering. They appreciate being with individuals, partaking in parties, and are brimming with vitality. An individual low in extraversion is less friendly and is progressively happy with working independent from anyone else.

People high in extraversion on a vocation test tend to search out the organization and incitement of other individuals.