The correct answer is: C. to compete successful with the rival firms
A firm under monopolistic competition advertises to create a brand identity and differentiate itself from its rivals. This can help the firm to attract more customers and charge a higher price.
Option A is incorrect because advertising does not usually lower the cost of production. In fact, advertising can be quite expensive.
Option B is incorrect because advertising does not always lead to increased sales and profits. In some cases, advertising can actually lead to lower sales and profits if it is not done effectively.
Option D is incorrect because a firm under monopolistic competition can raise the price, but it will not be able to raise the price as much as a firm under monopoly. This is because there are other firms in the market that offer similar products, so customers have more choices.