Answer: Option 'c' is correct.
Step-by-step explanation:
A barrier to entry is factor that makes it difficult for a new firm to enter a market .
As we can see in case of "Monopoly", there is difficult entry of new firms.
In "oligopoly", there is difficult entry of new firms but not as much as in monopoly.
New firms to enter in monopoly needs patents rights and legality from the government which is a very difficult task to destroy the monopoly of existing firm.
New firms to enter in oligopoly needs heavy investment which makes it difficult to enter.
Hence, Option 'c' is correct.