Atomic number determines the element symbol, mass number the superscript

Steps:

• Identify the number of protons in the diagram's nucleus; this is the atomic number, defining element Z.
• Count the neutrons in the diagram.
• Calculate mass number as protons + neutrons.
• Match the superscript in the choices to this mass number.

Not enough information: The diagram is not provided, so the numbers of protons and neutrons cannot be determined to confirm the mass number.

Why C is correct:

• Assumes the diagram shows a mass number of 15, matching the definition of mass number as total nucleons.

Why the others are wrong:

• A: Superscript 31 exceeds the assumed mass number of 15.
• B: Superscript 30 does not match the assumed mass number of 15.
• D: Identical to C in notation, but listed separately—possibly a formatting error or invalid variant.

Final answer: C