Equimolar solutions of the following substances were prepared...

$\mathrm{BaCl}_2$ is a salt of strong acid $\mathrm{HCl}$ and strong base $\mathrm{Ba}(\mathrm{OH})_2$. So, its aqueous solution is neutral with $\mathrm{pH} 7$. All other salts give acidic solution due to cationic hydrolysis, so their $\mathrm{pH}$ is less than 7. Thus, $\mathrm{pH}$ value is highest for the solution of $\mathrm{BaCl}_2$.