Option 1 -> Inviolable means sacred or unbreakable, unrelated to sleep or drowsiness.
Option 2 -> Wakeful means alert and unable to sleep, which is the opposite of soporific.
Option 3 -> Sophisticated means refined or complex, not related to inducing sleep.
Option 4 -> Somnolent means drowsy or sleep-inducing, directly synonymous with soporific.
Hence, Option 4: Somnolent -> Both "soporific" and "somnolent" refer to causing or being in a state of drowsiness or sleepiness, making them synonymous -> correct