The correct option is
C sad
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: We need a word to describe the sound of the pronoun. Since, 'you' is a pronoun, the word that describes it is called an adjective. Hence, we need an adjective to fill in the blank. (In such sentences, a person's 'look', 'sound', or any other thing is described using an adjective.)
Option A - 'Sadness' is a noun and not an adjective. Thus option A is incorrect.
Option B - 'Sadly' is an adverb and not an adjective. Hence, option B is incorrect.
Option D - 'Sadfully' is a wrong word. Thus option D is incorrect.
Option C - 'Sad' is an adjective which can be used to describe how one sounds. Hence, option C is the correct answer.
'What happened? You sound sad today.'