indeed better use logic with this kind of question; when I saw all the AA; I was about to have a heart attack )CFTR - Cystic fibrosis transmembrane conductance regulator. Mutation in CFTR leads to cystic fibrosis. In stem no word about CF.
Now only pure logic - we know that deletion of Phe in 508 leads to CF. From present choices, only C has Phe inside. So, assuming that no mutation present in transmembrane segment, what else we have to do, --> Phe is present --> choice C. May be they are non-polar on second look, but pure logic more fast .
I don't know exact order in CFTR and don't want to know, because in this segment are amino acids from 433 to 586