If you were thinking about starting up your own new firm, the industry you would find easiest to enter would be

a. monopoly, because there is only one firm in the industry.

b. oligopoly, because firms have considerable market power and therefore are less concerned about you entering.

c. monopolistic competition, because firms have differentiated products, which means you can enter to compete with a product of your own.

d. perfect competition, because there is free entry, meaning no barriers to entry.

e. unclear, because all have barriers to entry that you can overcome, so it really depends on the circumstances particular to the good you intend to produce.