Nuclide notation defines atomic and mass numbers

Steps:

• Identify the atomic number of cobalt (Co) as 27 from the periodic table, which equals the number of protons.
• Recognize the superscript 59 as the mass number (A), the total of protons and neutrons.
• Calculate neutrons as A minus atomic number: 59 - 27 = 32.
• Confirm the nucleus composition: 27 protons and 32 neutrons.

Why A is correct:

• Protons equal the atomic number (Z=27 for Co), and neutrons = A - Z = 59 - 27 = 32, per nuclide notation.

Why the others are wrong:

• B: Option incomplete or missing, cannot evaluate.
• C: Confuses mass number with protons; protons are Z=27, not 59.
• D: Mass number is not protons; protons remain Z=27, not 59.

Final answer: A