Answer : C
Explanation:
In the sentence, we have two subjects, i.e, Barbara & Sarah.
The construction of the sentence and the options give us a good idea that a comparison is being made between the two subjects.
Thus to correctly complete the sentence we are supposed to fill the blank with the comparative degree of the adjective:
--"more" is the comparative degree adjective of "much", which grammatically fits in the blank correctly. Thus it is the most appropriate from the options provided.
Also, when an adjective has three or more syllables, the comparative degree of the adjective is formed with "more"; and superlative degree with "most".
--"most" is the superlative degree of "much". It indicates a comparison between more than two things and hence, it is not the appropriate answer for this question. This eliminates option A.
--"much" means to a greater amount, but it is the base form and it does not indicate comparison.
--"intelligent" is just an basic adjective, thus it does not fit in here. This eliminates option D.
Therefore the correct complete sentence is:
Barbara's MORE INTELLIGENT than Sarah.