The correct answer is option (D):
natural
The correct answer is natural. The word "synthetic" means something that is made by chemical synthesis, especially to imitate a natural product. It's man-made. Therefore, the word that is opposite in meaning is "natural", which means existing in or caused by nature; not made by humans. Cosmetic, artificial, and plastic are all examples of synthetic items or related to synthetic processes, making natural the best antonym.