Which of the following pairs cannot be mixed together to form a buffer solution? WHY?

The answer is NaCl/HCl. Remember that a buffer is a mixture of a weak acid and its conjugate base (or equivalently a weak base and its conjugate acid). It resists changes in pH because the acid can neutralize added base and the conjugate base can neutralize added acid. It can be formed either by directly combining the acid and conjugate base (as in cases 1 and 5), or by partial neutralization of a weak acid with a strong base (as in cases 3 and 4) or a weak base with a strong acid. However, HCl is a very strong acid, which dissociates completely in water to form H+ (really H3O+) and Cl-. Cl- is therefore such a weak base that it will not absorb added H+ to resist changes in pH, which a buffer must be able to do.

It's NaCl and HCl, because there is no way that they can be combined to form either a weak acid/conjugate base or weak base/conjugate acid pair.

In response to the previous reply, RbOH and HF can form a buffer. If there is more HF than RbOH, then the hydroxides will pull the H+ of all the HF it can, forming water and F-. The rest of the HF will remain associated. Thus, you have HF (a weak acid) and F- (a conjugate base) in solution.

RbOH,HF doesn't work