Assertion (A) : Diamond is a good conductor of electricity. Reason(R): Diamond and graphite are two allotrope of carbon and graphite is a good conductor of electricity.

Assertion (A) is false because diamond is an electrical insulator, not a good conductor. In diamond, each carbon atom is sp3 hybridized and bonded to four other carbon atoms in a rigid three-dimensional tetrahedral structure [t2, t8]. All valence electrons are localized in strong sigma bonds, leaving no free or delocalized electrons to carry an electric current [t1, t5, t6]. Reason (R) is true; diamond and graphite are indeed allotropes of carbon [t1]. Unlike diamond, graphite is a good conductor of electricity because each carbon atom is sp2 hybridized, bonding to only three others [t2, t8]. This leaves one unhybridized p-orbital per carbon atom, creating a network of delocalized pi electrons that are free to move throughout the hexagonal layers [t2, t3, t5]. Since (A) is false and (R) is true, option 4 is the correct choice.