This article discusses the concept of a monopolist firm and its impact on allocative efficiency. It explains how a monopolist firm can maintain inefficient resource allocation and produce a lower amount of goods than the efficient quantity. The article also explores the advantages and disadvantages of a single seller system, highlighting the lack of competition and its impact on efficiency. Additionally, it discusses the conditions for allocative efficiency in perfect competition and monopolistic competition. Finally, it examines potential solutions to pollution-related negative externalities, such as carbon tax and quota-based emission systems.