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Answer:

C. NAD⁺  

Step-by-step explanation:

NADH is oxidized to NAD⁺ in Complex I of the Electron Transport Chain.

NADH ⟶ NAD⁺ + H⁺ + 2e⁻

The electrons continue through the Electron Transport Chain, and the NAD⁺ is used in three places during the Krebs Cycle.

(a) Isocitrate to oxalosuccinate

Isocitrate + NAD⁺ ⟶ oxalosuccinate + NADH + H⁺

(b) α-Ketoglutarate to succinyl-CoA

α-ketoglutarate + NAD+ + CoA → succinyl CoA + CO₂ + NADH

(c) Malate to oxaloacetate

Malate + NAD⁺  ⟶ oxaloacetate + NADH + H⁺

The NADH produced by these three reactions can then be used by Complex I in the Electron Transport Chain.

Basic info:

The Krebs cycle, the electron transport chain and respiration (aerobic) are closely related, in fact, the Krebs cycle is part of cellular respiration.

Further info (detailed):

In anaerobic respiration we obtain oxygen, which functions as an electron acceptor in the electron transport chain, which produces NAD+ that is later used in the Krebs cycle to produce NADH and FADH2 that are used as electron carryiers in the ETC.

But, what does the ETC actually do? Why is it so important?

In the ETC, basic substances are produced -  water, ATP and NAD/FAD.

The water and the ATP will be used in many random processes, but the NAD and the FAD are for the Krebs cycle (also glycolysis). During the Krebs cycle, NAD and FAD are used mainly for oxidizing acetyl-CoA to generate energy in the form of NADH and FADH2.

Why does the energy produced by the Krebs cycle come in the form of NADH and FADH2?

Beacuse during the oxidisation of acetyl-CoA (and others) both NAD and FAD react with H+.

Hope it helped,

BioTeacher101