Carbons forms many allotropes. Which of the given allotropes of carbon is the hardest? 

Diamond 

The correct answer is option 1. Diamond.

Among the given allotropes of carbon, Diamond is the hardest. Diamond is composed of carbon atoms arranged in a three-dimensional network structure, where each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement. These strong covalent bonds result in a tightly packed and rigid structure, making diamond the hardest naturally occurring substance known. Therefore, the correct answer is Diamond.