CH
3COOH and HCN are the stronger acids.
The pK
a values for CH
3COOH compared to HPO
4-2 is much lower making it a stronger acid. pK
a values for HCO
3- and HCN are extremely close, but HCN is a little bit less, again, making it a stronger acid.
pKa relates to the ability of an acid to dissociate causing it to become "stronger" or "weaker."