[bsaul]

I'd say it's an example of misphrased questions that people will intuitively make sense of, and thus produce strictly incorrect answer.

Obviously, people rephrase the question into : is there more chance that she's a feminist or that she isn't ?

[SAI_Peregrinus]

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.