All metals cannot be obtained by carbon reduction.
Carbon is poor reducing agent.

Options

A. If both (A) and ( R) are correct and ( R) is the correct explanation of (A).
B. If both (A) and ( R) are correct, but ( R) is not the correct explanation of (A).
C. If (A) is correct, but ( R) is incorrect.
D. If (A) is incorrect, but ( R) is correct.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "All metals cannot be obtained by carbon reduction." This means that there are certain metals that cannot be extracted from their ores using carbon as a reducing agent.

2. **Analyzing the Reason**: The reason provided is that "carbon is a poor reducing agent." To evaluate this, we need to understand the role of carbon in reduction reactions.

3. **Role of Carbon in Reduction**: Carbon is commonly used as a reducing agent in metallurgical processes. It can reduce many metal oxides to their respective metals. For example, when zinc oxide (ZnO) is heated with carbon, zinc is produced, and carbon monoxide is formed. This shows that carbon can effectively reduce certain metal oxides.

4. **Limitations of Carbon as a Reducing Agent**: However, carbon cannot reduce all metal oxides. For instance, aluminum oxide (Al2O3) cannot be reduced by carbon because aluminum is more reactive than carbon. In this case, carbon cannot displace aluminum from its oxide.

5. **Conclusion**: Therefore, the assertion is correct because not all metals can be extracted using carbon reduction. However, the reason is incorrect because carbon is not a poor reducing agent; it is a good reducing agent for many metals, but it has limitations based on the reactivity of the metals involved.

6. **Final Answer**: The assertion is correct, but the reason is incorrect.