Final answer:
To determine if two atoms or ions are isoelectronic, they must have the same number of electrons. This characteristic is independent of their protons, neutrons, or mass numbers.
Step-by-step explanation:
To identify when two atoms or ions are isoelectronic, you must look for a matching number of electrons. This occurs when two species have the same number of electrons regardless of the number of protons, neutrons, or their mass numbers. Therefore, the correct answer is c. They have the same number of electrons.
An atom is defined by its unique number of protons in the nucleus (atomic number), while an ion is an atom that has gained or lost electrons, resulting in a charge. Isotopes of an element have varying numbers of neutrons but identical numbers of protons and, in a neutral state, electrons. The mass number of an isotope is the sum of its protons and neutrons.