Final answer:
A successful advertising campaign can cause a firm's demand curve to shift to the right in monopolistic competition, increasing the demand for the firm's product. The correct answer is d. the demand curve shifts to the right.
Step-by-step explanation:
In the framework of monopolistic competition, a successful advertising campaign can cause the firm's perceived demand curve to shift to the right. This means that the demand for the firm's product increases. In other words, a successful advertising campaign can lead to an increase in the quantity demanded and potentially allow the firm to charge a higher price.
For example, if a firm launches a new advertising campaign promoting the benefits and features of their product, it can attract more customers and generate more demand. As a result, the firm's demand curve shifts to the right.
Therefore, the correct answer is d. the demand curve shifts to the right.