The correct answer is: "self-interest".
Consumers decide whether to consume a certain good of service, based on their individual preferences and on their budget, compared to the price charged for a specific product. Therefore, they would decide based solely on self-interest.
Self-interest was the base of the theories developed by the classical economist Adam Smith, who considered that the economy functioned efficiently when agents were left alone, motivated by their self-interest. Nowaydays, there are more modern theories which identify other factors that drive individual decisions, not only self-interest.
