"Its mass stays the same, and it becomes a different element" happens if an atom loses a neutron.
Option: D
Explanation:
Neutrons have a neutral charge, and are component of the nucleus, which means they contribute mass / weight ultimately to the atom. If you were to remove one of these, It would be the same aspect. Elements are clearly described by their protons and not by neutrons.
Neutrons do not carry an electrical charge and adding or removing it from the nucleus does not alter the nucleus' electrical charge. However it affects the nucleus mass. This isotopes are produced is the addition of or removal of neutrons from the nucleus.
