Correct Answer - 3
Each closest packed layer of spheres is formed by placeing spheres in the crevices at the adjoining row when a spher is placed in a hollow on top of a layer three other hollows are partially covered ,there are now two types of hollows on top of the second layer ,labeled `x` and `y` . The `x` sites are not . Sphere of a third layer may occupy either all of the sites or all of the `y` sites.
When the spheres in the third layer are placed in the `x`-sites so that the third layer repeats the first layer we label this stacking `ABA`. When successive layers are placed so that spheres of each layer are directly over a layer that is one layer away we get a stacking that we label `ABABABA....` and the result is a hexagonal closest packed structure.
Thus in hexagonal closest -packing there are two alternating hexagonal layers `A` and `B` offset from each other so that the spheres in one layer sit in the small triangular depressions of neighbouring layers.
