Which statement is true about an effective counterclaim?

it strengthens your argument.
it strengthens your opponent's argument.
it is presented in an introduction.
it is based on opinion rather than research.

It depends on your own argument. For A, It strengthens your argument, this would be true if the information in the paper was stronger than the counterclaim given.
For B, It strengthens your opponent's argument, this would be correct if the information in the paper is equivalent or almost close to the information to make both the counterclaim and your argument strong. 
For C, It is presented in an introduction, it would not necessarily be an introduction, but I usually put it right after the introduction because my body paragraphs will back up and support your claim. 
For D, It is based on opinion rather than research, sometimes this can be true, but it is good to look up opposing articles.

To narrow down the infinite amount of possible answers, I believe the answer they are looking for is "A". But it could also be "C.", but I personally think the answer is "A".

A counter claim will most definitely strengthen your argument. A counter claim is basically defending an attack that hasn't yet happened. With an argumentative essay, the people will try to find why your claim is wrong. They will seek weak points and missing pieces. Its smart to mention some counter claims so you can control how these counter claims have no affect on your claim. 

A counter claim is, in fact, presented in the introduction. Just like your claim, counterclaim is immediately introduced to be concise and to point direction in what you are trying to disprove basically.

Im leaning toward the answer "A" because they specially asked what is true about an effective counter claim. I believe that very word suggests, "how is an effective counter claim effective?"

It could very well be "C" in technicalities but they don't necessarily have to be presented in the introduction to be effective.

My answer would be "A".