Final answer:
The superlative adjective that goes with the sentence is 'Best'. Therefore, the correct option is c) Best.
Step-by-step explanation:
The superlative adjective that goes with the sentence 'The contralto singer is the ____________' is option c) Best. The word 'best' is used to describe the highest degree or quality of something. In this case, it is used to describe the contralto singer as the finest or most excellent in comparison to others. For example, if we say 'Mary is the best singer in the choir,' it means she is better than all the other singers.