Statement (A): Among the carbon allotropes, diamond is an insulator. Whereas, graphite is a good conductor of electricity.
Reason (R): Hybridization of carbon in diamond and graphite are $S{p}^2$ and $S{p}^3,$ respectively.

Hybridization of carbon in diamond is $S{p}^3.$ In diamond four valency electrons are participate in bonding so it is bad conductor of electricity. In graphite carbon hybridization is $S{p}^2.$ Out of four electron, three electrons are participated in bonding.The fourth free electron is participating in delocalisation and increase the conductivity.