Option 1 -> Doubtful means uncertain or questionable, which relates to uncertainty rather than sadness.
Option 2 -> Joyful means full of happiness and cheer, which is the opposite of lugubrious.
Option 3 -> Melancholy means sad, gloomy, and depressed, which directly aligns with the meaning of lugubrious.
Option 4 -> Thorough means complete or comprehensive, which is unrelated to emotional tone.
Hence, Option 3: Melancholy -> Lugubrious describes something that is mournful, dismal, or excessively sad in appearance or tone. Melancholy perfectly captures this sense of sadness and gloominess, making them synonymous -> correct