Which of the following is a true distinction between fermentation and cellular respiration?

Options

A. `NAD^(+)` functions as an oxidising agent only in respiration
B. Substrate-level phosphorylation is unique to fermentation
C. Fermentation,not respiration, is an example of a catabolic pathway
D. `NADH` is reduced by electron transport chain only in respiration

Explanation

To determine the true distinction between fermentation and cellular respiration, we can analyze the provided options step by step:

1. **Understanding Fermentation and Cellular Respiration**:
- Fermentation is an anaerobic process that occurs in the absence of oxygen, while cellular respiration is an aerobic process that requires oxygen.
- Both processes are involved in breaking down glucose to produce energy, but they differ in their mechanisms and end products.

2. **Evaluating the Options**:
- **Option 1**: "NAD functions as an oxidizing agent only in respiration."
- This statement is false. NAD is reduced to NADH in both fermentation and respiration, so it is not exclusive to respiration.

- **Option 2**: "Substrate level phosphorylation is unique to fermentation."
- This statement is also incorrect. Substrate-level phosphorylation occurs in both fermentation and cellular respiration, so it is not unique to fermentation.

- **Option 3**: "Fermentation, not respiration, is an example of a catabolic pathway."
- This statement is false. Both fermentation and respiration are catabolic pathways, as they both involve the breakdown of molecules to release energy.

- **Option 4**: "NADH is reduced by the electron transport chain only in respiration."
- This statement is true. The electron transport chain is involved only in aerobic respiration, where NADH is oxidized back to NAD+ as it donates electrons. In fermentation, there is no electron transport chain involved.

3. **Conclusion**:
- The correct distinction between fermentation and cellular respiration is found in option 4: "NADH is reduced by the electron transport chain only in respiration."

### Final Answer:
The true distinction between fermentation and cellular respiration is that NADH is reduced by the electron transport chain only in respiration.