The Figure Below Shows the Revenue and Cost Curves of a Monopolistically

The figure below shows the revenue and cost curves of a monopolistically competitive firm. Figure 25.2 In the figure,
D: Demand curve
MR: Marginal revenue curve
ATC₁ and ATC₂: Average total cost curves
MC: Marginal cost curve
In Figure 25.2, if the market is monopolistically competitive, which quantity represents long-run equilibrium for the firm?

A) 15
B) Between 15 and 40
C) 40
D) 55
E) 60