Final answer:
Expert power (option d) is considered the most effective base of power for positively influencing follower commitment, motivation, performance, satisfaction, and group effectiveness, because it is built on valued knowledge and skills.
Step-by-step explanation:
Research suggests that out of the given bases of power, expert power is the most effective in terms of its impact upon follower commitment, motivation, performance, satisfaction, and group effectiveness. Expert power is derived from having knowledge and skills that are valued. This type of power is often considered the most effective because it is built on respect for the expertise and persuasive abilities of the individual, rather than coercion or reward, which can lead to temporary compliance but not long-term commitment.
Legitimate power, often tied to a person's role or position within an organization, can also be effective, but it is dependent on the belief that the person has a rightful authority to wield power. Coercive and reward powers can lead to external compliance, but do not necessarily build intrinsic motivation or satisfaction. Position power identifies with having authority based on one's role in an organization yet does not inherently ensure follower commitment.