The correct option is
A sad, sadly
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 state of the little boy, and the second blank requires an adverb to describe the way he looked at 'me'.
Option B - 'Sadly' is an adverb and hence can't be used to fill the first blank. 'Sad' is an adjective and hence it can't be used to fill the second blank. Thus option B is incorrect.
Option C - 'Sad' is an adjective and hence it can't be used to fill in both the blanks as we require an adverb for the second blank. Hence, option C is incorrect.
Option D - 'Sadness' is a noun and 'sadly' is an adverb. As we need an adjective and not a noun, we can't use 'sadness' here. Hence, option D is incorrect.
Option A - 'Sad' is an adjective which can be used to describe the state of the little boy. 'Sadly' is an adverb which can be used to describe how he looked at 'me'. Hence, option A is the correct answer.
'I can tell that the little boy was sad because he looked at me sadly.'