The metal which can form an oxide having metal : oxygen atoms ratio 2 : 3 is

Assume that metal, M is there and charge on O is -2. 

Ratio of atoms in M and O is 2 : 3. This means that minimum value of O atoms is 3. Hence, charge on O atom is -6.

In Bismuth, Bi there are three valence electrons and for minimum value of Bi is 2 and valence electrons becomes +6. This nullify the charge of 3 O atoms, which is -6.