Which of the following is NOT a strong acid?

HF. The strength of the acid depends on the stability of the conjugate base. For HF, it's the F- anion, for HCl, it's Cl-, for H2SO4, it's HSO4-, etc. With the halides you can see the stability of the anion increases going down the periodic table. The reason is that the radius increases, helping spread out the negative charge. For this reason, HF is not a strong acid, but HCl is, and HBr is even stronger.