Two dimension close packed structures, where rows of identical spherical molecules are stacked on top of each other, can be done in two ways, square and hexagonal. In square close packed structure, the second row is stacked just above the first row, and the spheres are aligned horizontally and vertically as the way mentioned above. If we observe a sphere in this arrangement, it is surrounded by four spheres which are in direct contact with it. Joining of the centers of the four spheres also forms a square, thus giving the term square close packed structure. Thus by definition of coordination number, a sphere in this structure is 4.

The correct number is (iii).