Option 1 -> Flourish means to thrive, grow vigorously, or prosper - the direct opposite of decline or weaken.
Option 2 -> Yearn means to long for or desire something deeply - unrelated to the concept of declining.
Option 3 -> Marvel means to be filled with wonder or amazement - not connected to weakness or decline.
Option 4 -> Recoil means to draw back in fear or disgust - doesn't relate to the opposite of languishing.
Hence, Option 1: Flourish -> "Languish" means to grow weak, lose vitality, or suffer in unfavorable conditions, while "flourish" means to thrive, prosper, and grow strong - making them perfect antonyms -> correct