A bivalent metal has 12 equivalent weight. The molecular weight of its oxide is

 $Equivalent weight=\frac{Molecular weight}{n-factor}$

So, Molecular weight= equivalent weight x n-factor 

Since it is a bivalent metal therefore it’s n-factor=2.

So, Atomic weight=12 $\times$ 2=24

Since it is bivalent ion it needs only one oxygen atom to form an oxide.

Molecular weight of the oxide = Atomic weight of metal+ Atomic weight of oxygen.

Therefore molecular weight of the oxide = 24+16=40.

Hence the correct option is (C).