This economics report provides a comprehensive analysis of four fundamental market structures: perfect competition, monopoly, oligopoly, and monopolistic competition. It explores the characteristics, advantages, and disadvantages of each structure, including the number of buyers and sellers, product differentiation, barriers to entry, and pricing strategies. The report then delves into a detailed case study of the Nepal Oil Corporation (NOC), examining its role as a state-owned monopoly in the petroleum market. It investigates NOC's market power, pricing practices, and the resulting consumer and producer surplus, while also addressing the inefficiencies associated with its monopoly status, such as shortages, corruption, and ineffective subsidization policies. The analysis includes an assessment of NOC's financial performance and the impact of its market position on overall economic welfare, including deadweight loss. The report concludes by discussing potential strategies for improving efficiency and addressing the challenges posed by NOC's monopoly.