Answer:
when the buyer operates in an industry where products are undifferentiated
Explanation:
When a buyer can switch easily between suppliers and the goods are similar (not differentiated), then the buyer has higher bargaining power. The smaller the number of buyers, the more bargaining power they will have. A monopsony is the extreme case where one large buyer controls the market, e.g. a large factory in a small town can set the wages of its employees.