Answer: A)diamond
An allotrope of carbon that is mostly used in jewellery is diamond.
Explanation:
Allotropes of an element are the different physical forms in which the element can exist. These allotropes will have surprisingly different physical properties. Carbon has various allotropes like diamond, graphite and fullerene.
They differ in their physical appearance as well as physical properties. Accordingly they have differing uses. Graphite is electrically conducting while diamond is a poor electrical conductor. Just like carbon many elements have varying number of allotropes.