Nuclear size is femtometer scale

Steps:

• Atomic diameter is approximately 10^{-10} m, based on electron cloud extent.
• Nucleus occupies a small central volume, with radius R ≈ 1.2 × 10^{-15} m × A^{1/3} for mass number A.
• For typical A ≈ 100, R ≈ 5 × 10^{-15} m, so diameter ≈ 10^{-14} m to 10^{-15} m.
• Order of magnitude rounds to 10^{-15} m.

Why A is correct:

• Nuclear radius formula R = R₀ A^{1/3} with R₀ = 1.2 fm yields diameters around 2–10 fm (10^{-15} m) for light to heavy nuclei.

Why the others are wrong:

• B: Matches atomic diameter (angstrom scale), not nuclear.
• C: Equals 1 cm, a macroscopic length irrelevant to subatomic scales.
• D: Equals 100 km, absurdly large for any microscopic structure.

Final answer: A