Final answer:
The main concern of those wanting U.S. neutrality was fear that President Franklin Roosevelt was leading them into an unwanted war. Option B is correct.
Step-by-step explanation:
The main concern of the portion of Roosevelt's audience that wanted the US to stay neutral in the conflict was that Roosevelt was trying to lead them into a war they didn't want or need. This belief was largely fueled by the advocacy of noninterventionist sentiments in Congress and the American public's own aversion to becoming entangled in another European conflict.
President Franklin Roosevelt, while publicly supporting a strong stance of neutrality, had private communications that showed a growing desire to aid Britain and France if a war with Germany ensued. However, the strict noninterventionist policies, encouraged by figures such as Senator Gerald P. Nye, reflected a deep-seated reluctance to involve the United States in foreign conflicts, particularly in light of World War I experiences and an ongoing commitment to aiding victims of aggression through industrial and economic support, without engaging in military action.
Noninterventionist sentiment in Congress and among the public fueled this concern despite Roosevelt's private inclination to aid Britain and France against aggressors.