Which of the following is not an allotrope of carbon?

Allotropes are different structural modifications of an element, that is, each of two or more different physical forms in which an element can exist.

Carbon is capable of forming many allotropes due to its valency. Well-known forms of carbon include charcoal, diamond, graphite, buckminsterfullerene and graphene.

Hence, Ethylene is not an allotrope of Carbon.