Answer:
C is the answer.
Explanation:
In this case, the question refers to the closing sentence, meaning that it must go at the end of the paragraph.
Option A would be a good idea if it refered to the beginning of the paragraph because it introduces a concept.
B is also incorrect because it is very informal and doesn't match the tone of the paragraph.
Option D is also incorrect because, similar to option A, it sounds more likely to belong to the beginning of the paragraph.
Finally, option C seems to provide a closure because it offers a solution and the language is the proper one.