The answer that makes a speaker's argument most convincing is:
A) Using relevant and sufficient evidence.
While researching the topic thoroughly (B) and including opinions (C) can enhance an argument, the strength of the argument primarily lies in the quality of the evidence provided. Providing a call to action (D) can be effective in motivating an audience, but it does not necessarily strengthen the argument itself. Relevant and sufficient evidence helps to support the claims being made and builds credibility with the audience.