The definition of isomers can be given as follows.
An isomer is a compound that has same molecular formula but different structural formulas.
According to the definition we can say;
Isomers have same molecular formula. So they would have same empirical formulas. Since the molecular formulae is same then the molar mass of isomers would be the same.
Only the spatial arrangement of compound will be different in isomers.
For example `Co(NH_3)_4Cl_2^+` has two different arrangements as shown in the attached image. Both arrangement has same molecular mass, Empirical formulae and molecular formula but only the arrangement is different.
So the correct answer would be C)