Thermodynamically the most stable forrm of carbon is

1. A

   Coal

   Coal claims to be the most stable carbon allotrope due to its abundance in nature. However, it is an impure and amorphous form of carbon, making it thermodynamically less stable than crystalline allotropes.

2. B

   Fullerenes

   Fullerenes claim to be the stable form because of their unique spherical molecular cage structure. Despite this specialized structure, they possess higher internal strain energy compared to graphite.

3. C

   Graphite

   Graphite claims to be the thermodynamically most stable form of carbon under standard conditions. This is because its standard enthalpy of formation is defined as zero, making it the reference state against which other carbon allotropes are measured, thus making it the key.

4. D

   Diamond

   Diamond claims to be the most stable form due to its extreme hardness and tightly bonded tetrahedral crystal lattice. Although kinetically very stable, it has a slightly higher standard Gibbs free energy of formation than graphite.

Summary. Official key is (c) Graphite. Thermodynamically, graphite is the standard reference state for carbon, meaning its enthalpy and Gibbs free energy of formation are defined as zero at standard ambient temperature and pressure. Other allotropes like diamond and fullerenes have higher internal energies and will eventually transform into graphite, although the rate of this conversion is extremely slow at room temperature. Coal is an impure mixture rather than a pure allotrope of carbon. Therefore, graphite remains the most stable form from a thermodynamic perspective.