The correct answer is Option B: A-I, B-IV, C-II, D-III.

Here's why:

The Electron Transport System (ETS) in cellular respiration involves a series of protein complexes embedded in the inner mitochondrial membrane. Each complex plays a specific role in transferring electrons and pumping protons across the membrane, generating a proton gradient that drives ATP synthesis.

Let's break down the matching:

1. ETS Complex I (NADH Dehydrogenase): This complex is responsible for accepting electrons from NADH, a reduced electron carrier generated in glycolysis and the Krebs cycle. It then pumps protons into the intermembrane space.
2. ETS Complex II (Succinate Dehydrogenase): This complex is directly involved in the Krebs cycle, accepting electrons from succinate and transferring them to the electron transport chain. It does not pump protons.
3. ETS Complex III (Cytochrome bC$_1$): This complex receives electrons from Complex I and II and passes them to cytochrome c. It also pumps protons across the membrane.
4. ETS Complex IV (Cytochrome C Oxidase): This complex is the final electron acceptor in the chain, receiving electrons from cytochrome c and transferring them to molecular oxygen (O$_2$) to form water (H$_2$O). It also pumps protons.

Therefore, the correct match is:

• (A) ETS Complex I - (I) NADH Dehydrogenase
• (B) ETS Complex II - (IV) Succinate Dehydrogenase
• (C) ETS Complex III - (II) Cytochrome bC$_1$
• (D) ETS Complex IV - (III) Cytochrome C oxidase