Neutral atom: electrons equal protons, nucleons total 9

Steps:

• Nucleon number A = protons + neutrons = 9 for the atom.
• Neutrality requires electrons = protons (atomic number Z).
• Identify element by Z; beryllium has Z=4, so neutrons = 9 - 4 = 5, electrons = 4.
• Option A matches: 4 protons + 5 neutrons = 9, 4 electrons = protons.

Why A is correct:

• Matches definition of neutral atom (electrons = protons) and nucleon number formula A = Z + neutrons = 9.

Why the others are wrong:

• B: 5 electrons exceed 4 protons, creating a negative ion, not neutral.
• C: 5 protons indicate boron (Z=5), but boron's standard A=11, not 9.
• D: 4 electrons fewer than 5 protons, creating a positive ion, not neutral.

Final answer: A