Answer:
The correct answer is option a.
Step-by-step explanation:
A monopolistic competition is the type of market structure were there are a number of firms who are supplying close substitutes. The barrier on entry of new firms and exit of existing firms is low. The firms face a downward sloping demand curve.
The monopolistic firms keep their prices higher than marginal cost because there is low barrier on entry, so other firms enter the market attracted by super normal profits.
Business stealing externality is involved because new firms increase supply and thus reduce price level and profits.