The correct option is A
hardIn order to fill in the blank with the most suitable adjective or an adverb, let's understand what adjectives and adverbs are.
Adjectives are the words that define/explain nouns or pronouns. For example: Roshni is pretty. Here, 'Roshni' is the noun and 'pretty' explains how she is, hence 'pretty' is the adjective.
Adverbs are the words that describe action words (verbs). For example: Ram fought bravely. Here, 'Ram' is the noun and 'fought' is the verb. 'Bravely' describes how he fought, hence it is the adverb.
NOTE: We need a word to describe the way of 'trying'. Since 'try' is a verb, the word that describes it is called an adverb. Hence, we need an adverb to fill in the blank.
Option B - 'Hardly' is an adverb, but it means rarely or in a hard way. In the given sentence, the adverb should mean that one should try with a great effort. Thus option B is incorrect.
Option C - 'Hardness' is a noun and not an adverb. Hence, option C is incorrect.
Option D - 'Hardfull' is a wrong word. Thus option D is incorrect.
Option A - 'Hard' is an adverb which can be used to describe the way one should try in order to succeed. Hence, option A is the correct answer.
'We must try hard in order to succeed.'