In which one of the following market situations the practice of price rigidity is found ?

The phenomenon of price rigidity (sticky prices despite moderate changes in cost or demand) is a hallmark of an 'Oligopoly market'. As explained by Paul M. Sweezy's Kinked Demand Curve model (1939), an oligopolist faces an elastic demand curve for price increases (rivals do not follow) and an inelastic demand curve for price cuts (rivals match cuts), creating a discontinuity in marginal revenue that discourages price changes.