(i)${\mathrm{CaCO}}_3$ ,(ii) ${\mathrm{MgSO}}_4$ ,(iii) ${\mathrm{BaCl}}_2$ , (iv) $\mathrm{Sr}{\left({\mathrm{NO}}_3\right)}_2$,(v) ${\mathrm{MgBr}}_2$  and(vi)  ${\mathrm{MgCl}}_2$ 

The oxoacid salts of group II elements from the above list are

Oxoacids are the acids that contain oxygen atoms in it. Like, ${\mathrm{HNO}}_3,{\mathrm{H}}_2{\mathrm{SO}}_4,{\mathrm{H}}_3{\mathrm{PO}}_4$, etc. $\mathrm{HCl}$ is not an oxoacid as it does not have oxygen in it.

${\mathrm{CaCO}}_3$ is formed from ${\mathrm{H}}_2{\mathrm{CO}}_3$, which is an oxoacid. ${\mathrm{MgSO}}_4$ is formed from ${\mathrm{H}}_2{\mathrm{SO}}_4$, an oxoacid. ${\mathrm{BaCl}}_2$ is formed from $\mathrm{HCl}$, not an oxoacid. $\mathrm{Sr}{\left({\mathrm{NO}}_3\right)}_2$ is formed from ${\mathrm{HNO}}_3$, an oxoacid.

Hence, ${\mathrm{CaCO}}_3, {\mathrm{MgSO}}_4, \mathrm{Sr}{\left({\mathrm{NO}}_3\right)}_2$ are oxoacid salts and $\mathrm{Ca}, \mathrm{Mg}$ and $\mathrm{Sr}$ are elements of group II.