# Among $FeSO_4.7H_2O(A),CuSO_4.5H_2O (B), ZnSO_4.7H_20(c),MnSO_4.4H_2O$(D), isomorphous salts are

$(a)\;A\;and\;C \\ (b)\;A \;and \;D \\(c)\;C\;and \;B \\(d)\;A \;and \;B$

Compounds having similar molecular formulae and identical crystal structure are called isomorphous.
$FeSO_4.7H_2O$ and $ZnSO_4.7H_2O$ have similar chemical formulae and hence are isomorphous.
Hence a is the correct answer.