The correct option is
B bright
In order to fill in the blank with the most suitable adjective and 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 child. Since, 'child' is a noun, the word that describes it is called an adjective. Hence, we need an adjective to fill in the blank.
Option A - 'Brightful' is a wrong word. Thus option A is incorrect.
Option B - 'Brightly' is an adverb and not an adjective. Hence, option B is incorrect.
Option C - 'Brightness' is a noun and not an adjective. Hence, option C is incorrect.
Option D - 'Bright' is an adjective which can be used to describe the way the child is. Hence, option D is the correct answer.
'She seems to be a very bright child.'