Answer:
Letter a is correct. A successful advertising campaign will cause the firm's demand to the right.
Explanation:
In this case, the most appropriate option is the letter a. For the added benefits of a successful advertising campaign in a monopolistically competitive company will shift the demand curve to the right. This is because a successful advertising campaign consequently retains and attracts new customers, which represents an increase in consumption of a certain good or service, and the shift of the demand curve to the right, as it will increase the target audience of the company in question.