You are correct in the statement that buffers are weak acids or weak bases, but they ALSO need the salt of that acid or base. In this case, you have the salt of the weak acid but you don't have the weak acid itself.
Potassium fluoride, KF is the salt of a weak acid (HF), and so to make a buffer, one needs to have present the weak acid also.
Hydrolysis of KF will be as follows: F^- + H2O ==> HF + OH-
In order to increase the [HF], acid can be added to push the reaction to the right.
H+ + F- ==> HF.
While this is NOT a conventional method of preparing this buffer, of all the choices given, HCl would be the only one that would result in the formation of the weak acid, HF.