The correct option is
C stupid, stupidly
In order to fill in the blanks with the most suitable adjective and 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: The first blank requires an adjective to describe the thing, and the second blank requires an adverb to describe the act of 'arguing'.
Option A - 'Stupid' is an adjective and hence can't be used to fill in both the blanks. Thus option A is incorrect.
Option B - 'Stupidity' is a noun and 'stupidly' is an adverb. As we need an adjective and an adverb to fill in the first and the second blanks respectively, we can't use 'stupidity, stupidly'. Hence, option B is incorrect.
Option C - 'Stupidly' is an adverb and 'Stupid' is an adjective. As we need an adjective for the first blank and an adverb for the second blank, we can't use 'stupidly, stupid' here. Thus option C is incorrect.
Option D - 'Stupid' is an adjective which can be used to describe the thing (to say). 'Stupidly' is an adverb which can be used to describe the way arguing. Hence, option D is the correct answer.
'That was the most stupid thing to say. You shouldn't argue stupidly.'