The correct answer is A. How, specifically, is global warming affecting the environment?
Step-by-step explanation:
A follow-up question refers to a question asked in response to the position, opinion or explanation provided by someone else in reference to a specific topic or issue. The main purpose of these type of questions is to get more information about the ideas of others and because of this follow-up questions should directly address the same topic and ask others to clarify, explain or elaborate an idea that was not clear of completely developed.
This implies, in the case presented in which a speaker states "Global warming has been proven to be bad for the environment" without evidence an appropriate follow-up question should address this point and ask for evidence and therefore "How, specifically, is global warming affecting the environment?" is the best question as it addresses the same topic and acts as a response to the speaker, also this ask for more information which is the purpose of follow-up questions.