A simple cubic (primitive) unit cell has particles only at:

A simple cubic unit cell has atoms only at its 8 corners. Each corner atom contributes $1/8$ to the unit cell (shared with 8 neighbouring cells), so the effective atoms per unit cell is $8 \times 1/8 = 1$. Body-centred (option D) adds one atom at the centre, giving 2 atoms per cell. Face-centred (option C) adds atoms at the 6 face centres, each contributing $1/2$, giving 4 atoms per cell.