In a cubic packed structure of mixed oxides, the lattice is made up of oxide ions, one fifth of tetrahedral voids are occupied by divalent ($X^{++}$) ions, while one-half of the octahedral voids are occupied by trivalent ions ($Y^{+++}$), then the formula of the oxides.

$X_4Y_5O_{10}$
Hence (C) is the correct answer.