Both Shell and Engen serve a continued competition despite having different constant marginal cost competition but do not constitute a) a monopolistic competitive industry.

A monopolistic market structure is characterized by one primary seller who offers goods or services to the public. Here, the competition is less due to the fact that one seller dominates the market. Engen and Shell serve a perfectly competitive market. This is because they all serve the same market with similar products. What will set each one of them apart is how they get to price their products and attract their consumers.