leo is comparing carbon 12 and carbon 14 atoms which particles make these isotopes have different mass numbers

Carbon-12 (or 12C) contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 amu (six protons and six neutrons). Carbon-14 (or 14C) contains six protons, eight neutrons, and six electrons; its atomic mass is 14 amu (six protons and eight neutrons).

Hope this helped :)

the answer is definitely c.