B. stygian. Luminous means emitting light, so the opposite is darkness.
A. stentorian means very loud.
B. incandescent is a synonym.
C. mellifluous means harmonious (flowing smoothly) or filled with something that sweetens.
D. stygian means dark or gloomy (from th eRiver Styx).
E. innocuous means harmless.
(this multiple choice question has been scrambled)