Long-run equilibrium for a monopolistically competitive firm where economic profits are zero results from:

A) rising marginal costs.
B) a perfectly elastic product demand curve.
C) relatively easy entry.
D) product differentiation and development.

In monopolistic competition, firms can enter and exit the market relatively easily. This means that if existing firms earn economic profits in the short run, new firms will be attracted to the market. As new firms enter, the demand curve facing each individual firm will shift leftward, reducing market share and decreasing the ability to charge a high price. This process will continue until firms are earning only normal profits in the long run, which is where economic profits are equal to zero. Therefore, relatively easy entry is the reason for long-run equilibrium with zero economic profits.