Distinguishing relative atomic, isotopic, and molecular masses

Steps:

• Define key terms: relative atomic mass (Ar) is weighted average for element; relative isotopic mass is for specific isotope ≈ mass number; relative molecular/formula mass sums Ar of atoms.
• Check A: Ca Ar=40.1, C=12.0, O=16.0×3=48.0; total=100.1, not 100.5.
• Check B: ^24Mg isotope has relative atomic mass equal to mass number 24.
• Check C and D: ^16O isotopic mass=16 exactly by approximation; O2 molecular mass=16×2=32.

Why B is correct:

• Relative atomic mass of isotope ^24Mg is 24, matching its mass number as the standard value for a pure isotope.

Why the others are wrong:

• A: Actual relative formula mass of CaCO3 is 100.1 using standard Ar values.
• C: Relative isotopic mass of ^16O is exactly 16 by definition in basic contexts.
• D: Relative molecular mass of O2 is 32, not 16.

Final answer: B