Answer:
D. 4, 2, 1, 3
Explanation:
Succinate, through complex II, and NADH, via complex I, release electrons to coenzyme Q ⇒⇒⇒ Coenzyme Q releases its electrons to complex III ⇒⇒⇒Coenzyme Q-cytochrome c reductase passes electrons to cytochrome ⇒⇒⇒⇒Cytochrome oxidase catalyzes the reduction of O2 to water