In which step of citric acid cycle, FAD⁺ is reduced to FADH₂?

• A

  Conversion of $\mathrm{\alpha }$ ketoglutarate to succinyl CoA

• B

  Conversion of malate to oxaloacetate.

• C

  Conversion of citrate to isocitrate.

• D

  Conversion of succinate to fumarate.