Explanation:
As the number of valence electrons in boron is three, it forms compounds that are electron deficient. Such electron-deficient molecules tend to accept a pair of electrons to achieve stable electronic configuration and thus, behave as Lewis acids.
The correct answer is Graphite. The change in enthalpy of carbon in diamond to graphite form is negative. It shows that graphite should be more stable than diamond. The melting point of graphite is more than diamond which implies that it is a thermodynamically most stable allotrope of carbon.
