The correct answer is: B. Small number of buyers and sellers.
Perfect competition is a market structure in which there are many buyers and sellers of a homogeneous good or service, and there is free entry and exit into the market. In a perfectly competitive market, firms have no market power, and prices are determined by the forces of supply and demand.
A small number of buyers and sellers would not be a feature of perfect competition because it would allow firms to have market power. Market power is the ability of a firm to raise prices above the competitive level without losing all of its customers. If there are only a few firms in a market, they may be able to collude and raise prices above the competitive level. This would harm consumers, as they would have to pay higher prices for goods and services.
The other options are all features of perfect competition. A large number of buyers and sellers ensures that no one buyer or seller has a significant impact on the market. Free entry and exit allows firms to enter the market if they see an opportunity to make a profit, and exit the market if they are not making a profit. Homogeneous goods ensure that all firms in the market are selling the same good or service, so consumers can choose the firm that offers the best price.