Answer: D. Reference group
Explanation: A reference group is usually used to make comparison. Generally, reference group could be an individual or a group of people which are used as a basis or standard for evaluating or comparing the behavior another individual or group. Individuals or groups being compared may to to emulate the belief, norms or attitude of the reference group.
Beverage companies employ attractive young ladies 'models' to serve as reference groups so that visitors will tend to be attracted or emulate their behavior or action so as to boost sales and increase revenue.