In Chemiosmotic Phosphorylation, What Is the Most Direct Source of Energy

Multiple Choice

In chemiosmotic phosphorylation, what is the most direct source of energy that is used to convert ADP + ᵢto ATP?

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