Why patent within a monopoly?

The answer is C because a patent means that the product cannot be reproduced elsewhere for a certain period of time. This is a reward for your R&D and is fair because R&D would be disincentivised (i.e. people will no longer be driven to research and develop) if you spent millions on developing a product and another company just copied your product which saves them lots of money. Hence a patent basically gives you monopoly power as there is no other firm that can reproduce your product, and hence anyone that wants your product in the time period that the patent exists can only buy it from you (and hence you can charge whatever price you want). I hope that makes sense xd

Lets look at the other answers:
A: It is true that a firm will employ additional workers, but this surely will just increase cost. Furthermore, how does this give them monopoly power? - Possible but doesn't seem the logical answer

B: This statement is true but this means that there will be higher costs and hence making the firm less competitive. This will definitely not give you monopoly power. - This is definitely the wrong answer.

C: As explained above, this seems the most logical answer.

D: The merger of two firms will give monopoly power because they will own a bigger percentage of the market and have greater market power, but since the question says that it is a large industry, we can safely assume that this merger will not have too big an effect and hence wont yield too much monopoly power.

Therefore, the verdict is that C is the most correct answer and is most likely to give a source of monopoly power, and hence C would be the answer of choice :)

Hope that helped xd

Your logic escapes me. The question is "which is the SOURCE of the monopoly power". The patent itself give them the monopoly for that invention for that country.

Were it not for the patent, anyone having the necessary chemistry skills could simply reverse-engineer the drug, make a knock-off and sell at a market price. So much for the monopoly.