The closest-packed layers in the face-centred cubic unit cell are perpendicular to

The closest-packed layers in the face-centred cubic unit cell are perpendicular to the body diagonal of the cell.

Face-centred cubic unit is a cell cube cell with the face at the centre (FCC) Atoms are found in the centre of each cube face and at each corner of the crystal lattice in an FCC unit cell. Only half of each atom, which is located at the face centre, is allocated to each of the two neighboring unit cells.

Hence the option D is correct.