The anions (A) form hexagonal closest packing and atoms (C) occupy only 2/3 of octahedral voids in
it. then the general formula of the compound is-
C) Hexagonal close packing contains 6 atoms per unit cell and hence the number of octahedral voids per unit cell is 6.
Hence, number of A atoms per unit cell=6 and number of C atoms per unit cell=6 x 2/3=4
The formula of ionic compound is given as simplest formula and hence formula is C2A3