Two or more forms of an element are called

Two or more forms of an element are called allotrope.

Explanation:

When there is more than one form of an element in same states it is termed as allotrope. Different may exhibit different chemical and physical properties.

The reason behind the existence of an element in more than one form is due to difference in bonding pattern of array of atoms with each other in different forms.

where's the diagram?

it's all four options.