The number of atoms associated with a single primitive cubic unit cell is

The number of atoms associated with a single primitive cubic unit cell is 1

The atoms are only present in the corners of the basic cubic unit cell. Each corner atom is shared by eight nearby unit cells. There are 4 unit cells in the layer below and 4 in the layer above (or below). A specific unit cell therefore only contains 1/8th of an atom.

Hence option A is correct