Option 1: Mournful -> This is a synonym of lugubrious (both mean sad/sorrowful), not an antonym.
Option 2: Lucrative -> Means profitable or money-making; unrelated to the emotional meaning of lugubrious.
Option 3: Luculent -> Means clear or easily understood; not the opposite of sad/dismal.
Option 4: Joyful -> Means full of happiness and cheer; the direct opposite of lugubrious (sad, mournful, dismal).
Hence, Option 4: Joyful -> Lugubrious describes something sad, gloomy, or mournful, while joyful represents happiness and cheerfulness, making it the perfect antonym -> correct