Well, it's multiple-choice. So the "a" answer excludes the "b" answer by convention, and thus implies she isn't a feminist. Rephrase the question to remove that

Well, it's multiple-choice. So the "a" answer excludes the "b" answer by convention, and thus implies she isn't a feminist. Rephrase the question to remove that implication and I expect far more people will pick the "a" answer.