Which of the following salts will give highest $pH$ in water?

The highest pH refers to the basic solution containing $O{H}^{-}$ ions. Therefore, the basic salt releasing more $O{H}^{-}$ ions on hydrolysis will give highest pH in water. Only the salt of strong base and weak acid would release more $O{H}^{-}$ ion on hydrolysis. Among the given salts, $N{a}_{2}C{O}_3$ corresponds to the basic salt as it is formed by the neutralisation of NaOH [strong base] and $H_{2}C{O}_3$ [weak acid]. $C{O}_3^{2-}+H_{2}O\iff HC{O}_3^{-}+O{H}^{-}$