If the same type of atoms are packed in hexagonal closest packing (HCP) and cubical closest packing (CCP) separately, then 

If the same type of atoms are packed in hexagonal closest packing (HCP) and cubical closest packing (CCP) separately, then density of HCP will be equal to CCP.

HCP unit cells have 6 atoms in them. Each CCP unit cell has four atoms. Tetrahedral voids are covered by the third layer of spheres in hcp (hexagonal closed packing), which is identical to the first layer. So the stacking might be shown as an a-b-a-b-a-b type.

Hence the option C is correct.