Which of the following substances has free electrons to conduct electricity?
Graphite is formed by carbon bonding in hexagonal arrays in which every carbon atom is covalently joined to three other carbon atoms. The layers of these carbon atoms are stacked over one another. Since all the carbon atoms in graphite are bonded to three other carbon atoms and the fourth electron is free to move the mobility of these delocalised electrons from all the carbon atoms between the layers helps in conducting electricity.
Carbon monoxide, diamond and coal do not contain any free electrons and hence they do not conduct electricity.
Therefore, the answer is Graphite.