Answer: D (both react with oxygen to produce carbon dioxide). Diamond and graphite are both allotropes of carbon, so both burn in oxygen to give carbon dioxide, CO₂. That shared chemistry is the property common to both.
A is wrong: only diamond has each carbon atom forming four covalent bonds. In graphite each carbon forms only three covalent bonds, leaving one delocalised electron per atom between the layers.
B is wrong: only graphite acts as a lubricant, because its layers slide over one another. Diamond is extremely hard.
C is wrong: only diamond is an electrical insulator. Graphite conducts electricity because of its delocalised electrons. So the structural and physical properties differ, but the combustion chemistry is identical, making D the property they truly share.
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