Option 1: Painful -> Similar meaning to insufferable; both describe something unpleasant or hard to bear.
Option 2: Deficient -> Means lacking or inadequate; not related to the concept of tolerability.
Option 3: Endurable -> Means bearable or tolerable; directly opposite to insufferable (unbearable).
Option 4: Suppressible -> Means able to be suppressed; refers to control, not tolerability.
Hence, Option 3: Endurable -> "Insufferable" means unbearable or intolerable (cannot be suffered/endured), while "endurable" means bearable or tolerable (can be endured), making them direct antonyms -> correct