In Step 4 of the Citric Acid Cycle, the Reduction

In step 4 of the citric acid cycle, the reduction of NAD⁺ to NADH is coupled to the generation of CO₂ and the formation of a high-energy thioester bond.The energy of the thioester bond is harnessed in step 5.What is the energy used for?

A) to generate a molecule of GTP
B) to generate a molecule of ATP
C) to generate a proton gradient
D) to generate a molecule of NADH