Allotropes Of Carbon

Step 1: Understanding the Concept:
Different allotropes of carbon have different thermodynamic stabilities under standard conditions.

Step 2: Detailed Explanation:
Graphite is the thermodynamically most stable form of carbon at room temperature and pressure. Diamond is metastable (kinetically stable) and will eventually convert to graphite, though the process is extremely slow. Coal is a mixture of various organic compounds and carbon, not a pure form. Fullerenes are less stable than graphite.

Step 3: Final Answer:
Graphite is the most stable form, which corresponds to option (D).