The correct answer is: C. Very high pressure and high temperature.
Diamonds are formed when carbon atoms are subjected to very high pressure and high temperature. This can happen naturally, deep within the Earth’s mantle, or it can be done artificially in a laboratory.
The high pressure and temperature cause the carbon atoms to bond together in a very specific way, forming a crystal structure that is very strong and hard. This is what makes diamonds so valuable and useful.
Option A is incorrect because very low pressure would not be enough to cause the carbon atoms to bond together in the correct way. Option B is incorrect because very high temperature would cause the carbon atoms to vaporize, rather than bond together. Option D is incorrect because both very low pressure and very low temperature would not be enough to cause the carbon atoms to bond together in the correct way.