Nuclear sizes are expressed in the unit:

Question

The ratio of the mass densities of nuclei of $ ^{40}\text{Ca} $ and $ ^{16}\text{O} $ is close to:

Answer 1

We match each unit to the physical quantity it measures.

Tesla is a unit of magnetic flux density, used for magnetic fields, not for length.
Newton is a unit of force, also not a length unit, so it cannot describe a size.
Angstrom is indeed a unit of length, equal to $10^{-10}$ m, but this scale is used for atomic and molecular sizes, which are about a hundred thousand times larger than a nucleus, so it is not the standard nuclear-size unit.
Fermi is defined as $10^{-15}$ m, exactly matching the scale of atomic nuclei, making it the accepted unit for expressing nuclear radii in nuclear physics.

So option D, fermi, is the correct unit for nuclear size.

Answer 2

We match each unit to the physical quantity it measures.

Tesla is a unit of magnetic flux density, used for magnetic fields, not for length.
Newton is a unit of force, also not a length unit, so it cannot describe a size.
Angstrom is indeed a unit of length, equal to $10^{-10}$ m, but this scale is used for atomic and molecular sizes, which are about a hundred thousand times larger than a nucleus, so it is not the standard nuclear-size unit.
Fermi is defined as $10^{-15}$ m, exactly matching the scale of atomic nuclei, making it the accepted unit for expressing nuclear radii in nuclear physics.

So option D, fermi, is the correct unit for nuclear size.

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