The correct answer is: D. both buyers and sellers.
Under perfect competition, there are many buyers and sellers of a homogeneous good or service. This means that no individual buyer or seller has any significant influence over the market price. The price is determined by the interaction of supply and demand, and both buyers and sellers must accept the market price.
If there were only one seller, they would have a monopoly on the market and would be able to set the price. They could charge a higher price than the competitive price, and buyers would have to pay it because there are no other sellers from whom to buy.
If there were only one buyer, they would have a monopsony on the market and would be able to set the price. They could pay a lower price than the competitive price, and sellers would have to accept it because there are no other buyers to whom to sell.
In both cases, the market price would be different from the competitive price and would be determined by the monopolist or monopsonist. However, under perfect competition, there are many buyers and sellers, so no individual buyer or seller has any significant influence over the market price.