Three spheres of the first layer and three of the second layer enclose a site at the centre in a closest packing arrangement. This site is called:

The triangular voids in the first layer are overlapped by the triangular voids in the second layer. The interstitial void, formed by the combination of two triangular voids of the first and second layer, is called an octahedral void because it is enclosed between six spheres, the centers of which occupy the corners of a regular octahedron. Hence, option C is correct.