Correct answer: X4Y3
In a cubic close-packed (ccp) lattice, there are 4 atoms of Y per unit cell. Since two-thirds of the tetrahedral voids are filled by X, and there are 8 tetrahedral voids in a ccp structure, this results in approximately 5.33 atoms of X. The simplest ratio of X to Y, when reduced, gives the empirical formula X4Y3.