(E)
A simple cubic crystal is the only unit cell mentioned that has, on average, 1 atom per unit cell. A simple cubic unit cell has 1 atom in each of the 8 corners of the unit cell, but only 1/8 of each of those atoms is ascribed to that particular unit cell. A face-centered unit cell has 4 atoms per unit cell; while a body-centered unit cell has 2 atoms per unit cell.