The correct answer is option 4: very high pressure and high temperature.

Polymorphism refers to the ability of a substance to exist in multiple forms or crystal structures. In the case of Carbon, graphite and diamond are examples of polymorphs. While both graphite and diamond are composed of carbon atoms, the arrangement of these atoms differs significantly, resulting in vastly different properties.

Diamond is known for its hardness and brilliance, making it a valuable gemstone. It is formed under high-pressure conditions found deep within the Earth`s mantle, with temperatures reaching thousands of degrees Celsius. The intense pressure and temperature cause the carbon atoms to bond together in a tetrahedral arrangement, creating a rigid crystal lattice structure.

On the other hand, graphite is soft and has a layered structure, making it ideal for pencil leads and lubricants. It is formed under less extreme conditions, with lower pressure and temperature. The carbon atoms in graphite are arranged in layers, bound weakly by van der Waals forces, which allow the layers to slide over each other easily.

Therefore, the correct option is 4: very high pressure and high temperature, as it accurately describes the conditions under which diamond is formed.