The correct option is B Hardest
An adverb is a word that modifies a verb, adjective or another adverb.
The given sentence is in the superlative degree because it compares one person "Arum" with a group of people "of all".
Option B: 'Hardest' is an adverb of the superlative degree, hence it's the most suitable option.
Options A and C: Hard and harder are in the positive and comparative degree, respectively. But the given sentence is in the superlative, hence A and C are incorrect.
Option D: 'Most hard' is incorrect in itself. 'Most' is generally added to adverbs that end in -ly. So, it isn't added to words like 'hard'.