Final answer:
The theme selected for an advertising campaign that highlights a product's unique appeal is known as the unique selling proposition (USP).
Step-by-step explanation:
A desirable, exclusive, and believable advertising appeal selected as the theme for a campaign is called the unique selling proposition (USP). The USP is a marketing concept that identifies a product's distinct and appealing factor which sets it apart from competing products.
Successful use of a USP can make a company's products more attractive and memorable to consumers, potentially leading to increased demand or allowing the company to charge higher prices. In the context of monopolistic competition, effective advertising strategies, such as a strong USP, can also affect a firm's perceived demand curve, making it more inelastic or shifting it to the right, thereby enhancing a product's profitability.