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 Nisha complained. Since 'complained' is a verb, the word that describes it is called an adverb. Hence, we need an adverb to fill in the blank.

Option A - 'Coward' is an adjective and not an adverb. Thus option A is incorrect.

Option C - 'Cowardsome' is a wrong word. Hence, option C is incorrect.

Option D - 'Cowardfully' is a wrong word. Thus option D is incorrect.

Option B - 'Cowardly' is an adverb which can be used to describe the way Nisha complained to the teacher. Hence, option B is the correct answer.

'Nisha complained to the teacher in a very cowardly way.'