6.
A:
Lactate dehydrogenase catalyses the oxidation of pyruvate to lactate to regenerate NAD+.
B:
Lactate dehydrogenase ensures mitochondrial NADH is oxidised
C:
Lactate dehydrogenase catalyses the reduction of pyruvate to lactate to regenerate NADH.
D:
Lactate dehydrogenase catalyses the reduction of pyruvate to lactate to regenerate NAD+.
The answer is: D