Final answer:
Monopolistically competitive firms use the marginal revenue equals marginal cost approach to maximize profits.
Step-by-step explanation:
The type of firm that uses the marginal revenue equals marginal cost approach to maximize profits is monopolistically competitive firms. Monopolistically competitive firms aim to find the level of output where marginal revenue equals marginal cost, which results in maximizing their profits. Perfectly competitive firms also use this approach, but since monopolistically competitive firms have some degree of market power due to product differentiation, they can set their own prices.