Reward power is the power persisted by any manager or superior position holding person to provide reward to employees and workers for their outstanding working so that it can influence them perform better in future. This reward can be in form of salary increment, bonus, promotion etc.
According to the question, reward power is used by supervisor for providing gift certificate to employee for his good job as a reward.It is for influencing his job in the project.
Other options are incorrect because expert power and referent power are not the measure for influencing any employee by providing rewards. Thus,the correct option is reward power.
