Which one of the following forms of carbon is thermodynamically most stable?

Among the allotropes of carbon, graphite is the thermodynamically most stable form at standard conditions (25 °C, 1 atm). Its standard enthalpy of formation is defined as zero by convention, and diamond has a positive enthalpy of formation (~1.9 kJ/mol) relative to graphite, meaning diamond is metastable and slowly converts to graphite (over geological time scales). Fullerenes and coal (impure form) are also less thermodynamically stable than graphite.