The correct answer is in parentheses after a sentence.
While signal phrases can appear at the beginning, end, or in the middle of a sentence, parentheses usually function as an aside or clarification within the main sentence. They wouldn't naturally introduce a counterclaim, which often stands in contrast to the previous point and requires clearer introduction.
Here's an example to illustrate:
Incorrect: My argument relies on the fact that... (However, some might argue that...).
Correct: My argument relies on the fact that... However, some might argue that...
Therefore, even though parentheses can contain opposing views, they don't function as proper introductions for counterclaims within an argumentative essay.