In Chemiosmosis, What Is the Most Direct Source of Energy

Multiple Choice

In chemiosmosis, what is the most direct source of energy that is used to convert ADP + ℗i to ATP?

A) energy released as electrons flow through the electron transport system
B) energy released from substrate-level phosphorylation
C) energy released from dehydration synthesis reactions
D) energy released from movement of protons through ATP synthase, down their electrochemical gradient
E) No external source of energy is required because the reaction is exergonic.