Which of the following following is smallest unit?

Options

A. Millimeter
B. Angstrom
C. Fermi
D. Metre

Explanation

To determine which of the given units is the smallest, we need to convert all the units into the same base unit, which in this case will be meters (m).

### Step-by-Step Solution:

1. **Identify the Units**: The units we need to compare are millimeters (mm), micrometers (μm), and Fermis (fm).

2. **Convert Each Unit to Meters**:
- **Millimeter (mm)**:
\[ 1 \text{ mm} = 10^{-3} \text{ m} \]
- **Micrometer (μm)**:
\[ 1 \text{ μm} = 10^{-6} \text{ m} \]
- **Fermi (fm)**:
\[ 1 \text{ Fermi} = 10^{-15} \text{ m} \]

3. **Compare the Values**: Now we have the following conversions:
- \(1 \text{ mm} = 10^{-3} \text{ m}\)
- \(1 \text{ μm} = 10^{-6} \text{ m}\)
- \(1 \text{ Fermi} = 10^{-15} \text{ m}\)

4. **Determine the Smallest Unit**: To find the smallest unit, we look at the powers of ten:
- \(10^{-3}\) (mm)
- \(10^{-6}\) (μm)
- \(10^{-15}\) (fm)

The smallest exponent (most negative) is \(-15\), which corresponds to the Fermi.

5. **Conclusion**: Therefore, the smallest unit among the given options is:
\[ \text{1 Fermi} \]

### Final Answer:
The smallest unit is **1 Fermi**.