Choice "C" is correct. Oligopoly market conditions are characterized by:

• Few firms in the market

• Significant barriers to entry

• Differentiated products

• Fixed (or semi fixed) prices

• Kinked demand curves

Choice "b" is incorrect. This is an example of monopoly.
Choice "d" is incorrect. Horizontal demand curves represent demand that is perfectly price elastic (buyers will only pay one price for any quantity of a product). This occurs in perfectly competitive markets.
Choice "a" is incorrect. This is characteristic of perfect competition, as there are no barriers to entry ("size doesn't matter") in perfect competition.