Explanation:

- Option 1: Micron

- A micron (or micrometer) is 1 micrometer, or 10^(-6) meters.

- It is much too large a unit for the atomic or subatomic scale.

- Option 2: Nanometer

- A nanometer is 10^(-9) meters.

- Still relatively large for nuclear measurements, usually used for atomic scale.

- Option 3: Fermi

- A fermi, or femtometer, is 10^(-15) meters.

- Perfect for expressing sizes like nuclear radii.

- This is the most appropriate unit for nuclear radius.

- Option 4: Angstrom

- An angstrom is 10^(-10) meters.

- Used for atomic-scale measurements, not small enough for nuclear scales.