Explanation

(d)
Graphite has a two dimensional structure. In this case, only three of the four valence electrons of each carbon atom are involved in bonding. Thus, each carbon atom makes use of $sp^{2}$-hybrid orbitals. Hence, the fourth valence of each carbon atom remains unsatisfied $ie$, the fourth valence electron remains unpaired or free. This free electron can easily move from one carbon to another under the influence of applied potential. So, in structure of graphite only one electron is free on each carbon atom.