Question

In which portion of the essay should you address a counterargument?

Answer 1

The answer is C. Body paragraph 3

Answer 2

Usually, the rule of thumb indicates the following:
1- Introduction
     You say what you are trying to do with your text and how you are trying to accomplish it.
2- Body [1]
     You introduce your topic and your argument [thesis] and introduce supporting detail.
3- Body [2]
     You elaborate on the supporting detail.
4- Body [3]
      You state what your opposing party believes regarding your argument, and refute it.
5- Conclusion
      You reflect on what you have stated to accomplish the objective mentioned in the introduction, and how you have accomplished it.

The reason why you should not address a counterargument before/after where is suggested is because it's counterproductive to the purpose of the segments in the structure. If you do it in the second paragraph of your body, you didn't allow yourself space to support your thesis. Similarly, the introduction and conclusion paragraphs are not there for you to address anything, so doing so would be incoherent.