`O_(2)` is used by -

Options

A. Citric acid cycle
B. Electron transport chain
C. Substrate level phosphorylation
D. ATP synthase

Answer

**Step-by-Step Solution:**

1. **Understanding Oxygen's Role in Respiration:**
- Oxygen (O₂) is primarily involved in aerobic respiration, which is the process that occurs in the presence of oxygen. It is important to note that oxygen is not used in anaerobic respiration.

2. **Identifying the Steps of Aerobic Respiration:**
- Aerobic respiration consists of several key steps:
- Glycolysis
- Link Reaction (also known as Pyruvate Oxidation)
- Citric Acid Cycle (Krebs Cycle)
- Electron Transport System (ETS)

3. **Determining Where Oxygen is Used:**
- In the context of aerobic respiration, oxygen is specifically used in the Electron Transport System (ETS).
- During the ETS, oxygen acts as the terminal electron acceptor. This means that it accepts electrons that have passed through various electron carriers.

4. **Understanding the Role of Oxygen in the ETS:**
- As electrons move through the electron transport chain, they release energy, which is used to pump protons across the mitochondrial membrane, creating a proton gradient.
- At the end of the chain, oxygen combines with these electrons and protons to form water (H₂O). This process is crucial for the continuation of the electron transport chain and for the production of ATP.

5. **Conclusion:**
- Therefore, the correct answer to the question "O₂ is used by" is that oxygen is used in the Electron Transport System as the terminal electron acceptor during aerobic respiration.