Length cannot be measured by

Options

A. fermi
B. micron
C. debye
D. light year.

Explanation

To solve the question "Length cannot be measured by," we need to analyze the given options one by one to determine which one is not a unit of length.

### Step-by-Step Solution:

1. **Understanding the Options**:
- We have four options:
- A) Fermi
- B) Micron
- C) Dewey
- D) Lightyear

2. **Analyzing Option A: Fermi**:
- A Fermi (symbol: fm) is a unit of length equal to \(10^{-15}\) meters. It is commonly used in nuclear physics to measure sizes at the atomic scale.
- **Conclusion**: Fermi is a unit of length.

3. **Analyzing Option B: Micron**:
- A micron (or micrometer, symbol: µm) is equal to \(10^{-6}\) meters. It is used to measure small lengths, such as the size of bacteria.
- **Conclusion**: Micron is a unit of length.

4. **Analyzing Option C: Dewey**:
- The term "Dewey" does not refer to a standard unit of length. In fact, it is associated with the Dewey Decimal Classification system used in libraries, which has nothing to do with measuring length.
- **Conclusion**: Dewey is not a unit of length.

5. **Analyzing Option D: Lightyear**:
- A lightyear is the distance that light travels in one year, approximately \(9.46 \times 10^{12}\) meters. It is used to measure astronomical distances.
- **Conclusion**: Lightyear is a unit of length.

6. **Final Conclusion**:
- From the analysis, we find that the only option that does not represent a unit of length is **Option C: Dewey**.

### Final Answer:
**Length cannot be measured by: C) Dewey**