C(Graphite)⇌C(Diamond); ∆H=+1.9kJmol–1 Favorable condition for getting a good yield of diamond (density of graphite and diamond are 2.3 g/ml, 3.5 g/ml)

Diamond has a density that is higher than that of graphite. As a result, as pressure increases, more and more diamonds will develop, causing volume to drop. Graphite-to-diamond conversion is also an endothermic reaction. It will therefore be preferred at high temperatures.

As a result, the creation of diamonds from graphite is favored by high pressure and temperature.

Therefore, the correct answer is an option (C) High temperature, high pressure.