The core concept of the question is to identify synonyms. The correct answer is 'B. Prosper' because 'flourish' and 'prosper' both mean to grow or develop successfully. 'A. Decline' means to decrease or go down in quality or quantity, 'C. Wither' means to dry up or decay, and 'D. Stagnate' means to stop developing or growing. These are all antonyms of 'flourish'.