The correct option is A that
The correct option is A. The relative pronoun 'that' is used in preference over 'Which' or 'Who', after adjectives in the superlative degree. Hence, Options B and C are incorrect. A Pronoun is a word or phrase used in place of a noun. A relative pronoun is a pronoun which is used to refer to a noun (any person, place, thing or idea) that was previously mentioned or heard of. When we use adjectives of superlative degree in a sentence, we should use 'that' to refer to them. An Adjective describes the qualities and attributes of a noun. A superlative adjective is used to compare three or more nouns. It takes the comparison of nouns to the highest form. Example: Big, bigger, and biggest. Biggest is a superlative adjective. Therefore, in the above sentence where a superlative adjective (biggest) is used, we should use 'that' to refer to it in preference to 'Which' or 'Who'.