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.

Option A - 'She behaves nice because she is a nice person.' Here, 'nice' is used as an adjective in both the clauses of the sentence. In the first clause of the sentence, we need an adverb to describe the way she behaves, whereas in the second clause 'nice' is correctly used as an adjective. Hence, option A is incorrect. 

Option B - 'She behaves nicely because she is a niceful person.' Here, 'nicely' is used in the first clause of the sentence as an adverb correctly. However, 'niceful' is a wrong word. Hence, option B is incorrect. 

Option D - 'She behaves nice because she is a nicely person. Here, 'nice' is used as an adjective instead of an adverb in the first clause and 'nicely' is used as an adverb instead of an adjective in the second clause. Hence, option D is incorrect. 

Option C - 'She behaves nicely because she is a nice person.' Here, 'nicely' is used as an adverb in the first clause of the sentence to explain the way she behaves, and 'nice' is used as an adjective in the second clause of the sentence to describe the pronoun 'she'. Hence, option C is the correct answer.