What defines the mass number of an isotope? the sum of the neutrons and protons the sum of the neutrons and electrons the number of neutrons the number of protons

the sum of the neutrons and protons

Explanation:

Mass number is defined as the sum of number of protons and neutrons that are present in an atom.  Protons and neutrons are located in the nucleus and contributes to the mass of an atom.

Mass number = Number of protons + Number of neutrons

Atomic number is defined as the number of protons or number of electrons that are present in an atom.

Atomic number = Number of electrons = Number of protons

Hence, sum of neutrons and protons define the mass number.

the answer is d.) potassium