Choice "D" is correct. The inability of market participants (a single trader in this instance) to influence market prices is an attribute of perfect (pure) competition. Attributes of perfect competition also include a large number of suppliers, customers acting independently, very little product differentiation (homogeneous products), and no barriers to entry exist.
Choice "b" is incorrect. Market participants cannot influence prices in perfectly competitive markets.
Choice "a" is incorrect. Trading prices are based on both supply and demand in perfectly competitive markets.
Choice "c" is incorrect. Pricing information is available to all market participants in perfectly competitive markets.
