Option 1 -> Opulence means wealth or luxury, unrelated to energy or vitality.
Option 2 -> Robustness means strength, vigor, and health - the opposite of languor (weakness/lethargy).
Option 3 -> Indolence means laziness or idleness - actually a synonym of languor, not an antonym.
Option 4 -> Apathy means lack of interest or emotion - similar in nature to languor, not opposite.
Hence, Option 2: Robustness -> Languor refers to a state of tiredness, weakness, or lack of energy, while robustness conveys strength, vigor, and vitality, making it the most appropriate antonym -> correct