An adverb is a word or phrase that qualifies or changes an adjective, verb or another adverb, to express a relation of a situation, cause etc.
The correct answer is B)miserably, unhappy as 'miserably' is an adverb which describes the adjective 'unhappy'.
Options A, C, and D are wrong as 'miserable' is an adjective which cannot be used in the place of an adverb and 'unhappily' is an adverb which cannot be used in the place of the adjective 'unhappy'.
The poor girl looked miserably unhappy.