Answer:
The answer to your question is:
Explanation:
Mass number is the sum of protons and neutrons if an atom.
Protons and neutrons in the atom. This is true is the definition of mass number.
Neutrons and electrons in the atom. It's false, if we sum the number of neutrons and electrons, the result could be the same as the mass number but it's not the definition of mass number.
Protons in the atom. Incorrect, if only consider protons that is atomic number not mass number.
Electrons in the atom. Incorrect, mass number doesn't consider electrons in its definition.
