Concept:A substance conducts electricity only if it contains free electrons or mobile ions.Explanation:Graphite conducts electricity because it has free delocalised electrons.Sulphuric acid (H2SO4) conducts electricity because it forms mobile ions.A table salt solution (NaCl solution) also conducts electricity due to the presence of free-moving ions.Glass is an insulator because its electrons are tightly bound and it has no mobile ions in the solid state.Therefore, glass is the only substance listed that does not conduct electricity.Answer:B. glass