Which of the following statement is correct?

Diamond is the metastable allotrope of carbon-based each C-atom forms a tetrahedral unit through covalent bonds with four other C-atoms. Each C atom has a ${\mathrm{sp}}^3$ hybrid.

Le Chatelier principle shows that diamond formation from graphite is encouraged under high pressure settings because when pressure increases, the equilibrium moves in that direction, leading to a reduction in volume or a rise in density.

Hence the correct option is a and c.