Among the following, relatively strong base is

Strong bases like the H- ion take protons and retain them so tightly that the conjugate acid H2 has no tendency to donate a proton. H2 is therefore a very weak acid.

$\therefore {\text{CH}}_{\text{3}}\text{COOH } \text{ is the weakest conjugate acid and}$

 $\text{ hence, } {\text{ CH}}_{\text{3}}{\text{COO}}^{\text{- } }\text{ is the strongest base}$

Hence, the correct option is option (C) ${\text{CH}}_{\text{3}}{\text{COO}}^{\text{-}}$.