Final answer:
The presence of a new coffee shop in a monopolistically competitive market typically decreases the sales of other individual shops due to increased competition and a shift in the demand curve to the left for existing firms. Option b is the answer.
Step-by-step explanation:
In a monopolistically competitive industry such as coffee shops, competition among sellers impacts the market dynamics significantly. The correct answer to how the presence of a new coffee shop affects the existing market is: b. the presence of a new shop causes a decrease in the sales of other individual shops. This is because the entry of other firms leads to a shift in the demand curve faced by existing firms. More specifically, their perceived demand curve shifts to the left, indicating a reduction in the quantity demanded at a given price. This shift is due to the increase in supply and similar products offered by new entrants, which disperses the customer base across more sellers and can decrease each firm's share of the total market.