How many unit cells share an atom that is located at the center of a cube edge of a unit cell?

zero

Explanation:

In a unit cell, an atom that is located at the center of a cube edge is not involved in sharing unit cells because a central atom of a unit cell belongs to the entire cell and only to that unit cell of the lattice.

Hence, the center atom of a unit cell do not share any unit cell and the correct answer is "Zero".

a