The correct answer is option (A):
rejoice
The correct answer is rejoice because "repine" means to express discontent or to feel or show dissatisfaction. The word "rejoice" means to feel or show great happiness; the opposite of expressing dissatisfaction. The other options, "repose," "repent," and "mourn" are not antonyms of "repine." Repose means to be in a state of rest, or to lie or be situated. Repent means to feel or express sincere regret or remorse about one's wrongdoing or sin. Mourn means to feel or show sorrow for the loss of someone or something. Therefore, rejoice is the best antonym.