Final answer:
The characteristic most closely related to leader effectiveness is high conscientiousness, as it involves competence, self-discipline, and goal-directed behavior, which are crucial for leadership success.
Step-by-step explanation:
The characteristic most closely related to leader effectiveness is high conscientiousness. Conscientious individuals display qualities such as competence, self-discipline, and a strong drive to achieve their goals, which are critical for effective leadership. Numerous studies have shown that conscientiousness has a positive correlation with various forms of success, including academic achievement and reliable performance in the workplace. While other traits like openness, extraversion, agreeableness, and neuroticism may also influence leadership to differing extents, conscientiousness consistently emerges as a significant predictor of effective leadership and organizational success.