Final answer:
Barriers to market entry are NOT part of a pure competition because they prevent new competitors from entering the industry, which contradicts the principles of perfect competition.
Step-by-step explanation:
The aspect that is NOT part of a pure competition is d) Barriers to market entry exist. In a perfectly competitive market, there are essential characteristics that must be present. These include many sellers and buyers participating in the market, the offering of identical products by sellers, well-informed sellers and buyers about products, and free entry and exit into the market by sellers. Barriers to entry prevent new competitors from easily entering an industry and contradict the principles of perfect competition, where there should be no barriers of entry or exit. Instead, barriers to entry can lead to monopolies or other market structures that do not represent perfect competition.