Question

diamond is ___hardest of all minerals use (a, an,the )​

Answer 1

Answer:

The will be the answer...

Answer 2

THE

Diamond is the hardest of all minerals.

The reason the is used is because the word that follows is hardest, a superlative degree of Hard. In Grammar, when a superlative degree is used, article the is used because the statement is most often, making a claim or stating a fact which is perhaps unchangeable or true. In order to express the sentence in its real sense, article the is useful.