Nucleus Composition in Beryllium-9

Steps:

• Beryllium (Be) has atomic number 4, so 4 protons in nucleus.
• Stable isotope is Be-9 with mass number 9, adding 5 neutrons.
• Total nucleons: 9, all baryons (protons and neutrons are baryons made of 3 quarks).
• Nucleus contains only baryons as hadrons; mesons (quark-antiquark pairs) are not stable in nuclei.

Why C is correct:

• Nucleus has 9 baryons (4 protons + 5 neutrons), all hadrons, with 0 mesons per standard nuclear model.

Why the others are wrong:

• A and B include mesons (4 or 5), but nuclei lack stable mesons.
• D duplicates C but is listed separately, likely an error; C is specified correct.

Final answer: C