Advertising can enhance economic efficiency when it:

A) increases brand loyalty.
B) raises entry barriers.
C) increases consumer awareness of substitute products.
D) boosts average total cost.

Advertising that increases consumer awareness of substitute products can lead to more informed purchasing decisions, encouraging competition and ultimately driving down prices. This can enhance economic efficiency by promoting market competitiveness and reducing market power or monopolistic behavior. On the other hand, options A, B, and D are not likely to enhance economic efficiency. Brand loyalty may reduce consumer willingness to switch to competing products, reducing competition and potentially leading to higher prices. Raising entry barriers may limit competition and reduce consumer choice. Boosting average total cost may increase prices and reduce consumer surplus.