Answer:
D. Decrease
Explanation:
Monopolistic Competitive market structure includes many sellers selling related but differentiated (imperfect substitutes) of each other.
This market has free entry & exit, firms' partial control over price-based on differentiation but also market competition, imperfect knowledge-based on selling cost & claimed superiority of all firms' products over each other.
Eg : Cosmetic, Skin care products.
If new firms enter this market , the existing firms demand curve becomes more elastic (more responsive to price change) & demand decreases , shifts leftwards.