a) $K_2O + H_2O \to 2KOH$
$CaO + H_2O \to Ca(OH)_2$
$SO_2 + H_2O \to H_2SO_3$
b)
$K_2O + 2HCl \to 2KCl + H_2O$
$Al_2O_3 + 6HCl \to 2AlCl_3 + 3H_2O$
$CaO + 2HCl \to CaCl_2 + H_2O$
$CuO + 2HCl \to CuCl_2 + H_2O$
$FeO + 2HCl \to FeCl_2 + H_2O$
$MgO + 2HCl \to MgCl_2 + H_2O$
c) $2NaOH + SO_2 \to Na_2SO_3 + H_2O$