Remember when ranking acids, you always want to think of the stability of the conjugate base. The conjugate base of the above choices would have S- instead of SH. Whichever conjugate base is more stable, it means the acid is stronger (stable conj. base = strong acid, unstable conj base = weak acid).
The correct answer has a Fluorine on the end because the Fluorine helps stabilize the negative charge of the conjugate base. Comparing it to choice A, you know that D is stronger because the fluorine is closer to the negative charge, so it will have a larger stabilizing effect. This is definitely something you want to become good at!
