Option 1: Joyful -> Means happy and cheerful, which is the opposite of plaintive.
Option 2: Peaceful -> Means calm and tranquil, unrelated to the sorrowful tone of plaintive.
Option 3: Mournful -> Means expressing sadness or grief, which directly matches the meaning of plaintive.
Option 4: Triumphant -> Means victorious and celebrating, which contrasts with plaintive.
Hence, Option 3: Mournful -> "Plaintive" describes a tone or sound that expresses sorrow, sadness, or melancholy, making "mournful" the closest synonym -> correct