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