Hi bjohnson2021!
Especially if you've narrowed it down to just two answer choices, for a point at issue question like this one, it can be useful to put an initial for each speaker next to the answer choice under consideration. Then, ask whether the speaker would agree with or disagree with the statement, or write down a question mark if it's unclear whether the speaker would agree or disagree. In this particular question, we're asked for a point of disagreement.
Answer choice (D) states, "one should speak out against polluting even if doing so reveals one’s own hypocrisy." We can note the views of each speaker on this:
P: Disagree
J: Agree
This confirms that this is a point of disagreement with them, and thus the correct answer choice. P would disagree with (D) (P concludes that we "cannot speak out ... We would be revealing our hypocrisy). J would agree with the statement (J concludes that "it would be worse not to speak out ... just because I am being hypocritical").
By contrast, answer choice (B) states, "speaking out against the use of gas guzzlers despite driving in situations in which one could use a bicycle reveals hypocrisy." Asking whether the speakers agree or disagree, we'd have:
P: Agree
J: Agree
P would agree with this statement ("We would be revealing our hypocrisy"). J would also agree with this statement ("just because I am being hypocritical"). Since they'd both agree, (B) is not a point of disagreement and thus not the correct answer choice.