Hi Faraah,
Be careful with your reading! The passage states "using wild type p27 and three mutants" this does not suggest that the wild type is not a variant. It is important to carefully consider choices before ruling them out, and using all the information in the passage and question to determine the best answer! In this case, A is the correct answer because In Figure 2, the sample with the lowest percentage of cells in S phase is the p27-KIDWT variant.
Hope that helps!