Final answer:
Nickel isotopes have the same atomic number but different mass numbers, due to variations in the number of neutrons, leading to the correct answer being C. different mass, same atomic number.
Step-by-step explanation:
The element nickel has five naturally occurring isotopes. Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons. Therefore, the relationship of these isotopes of nickel is that they have a different mass, but the same atomic number.
Atoms of an element that contain different numbers of neutrons are called isotopes. Each isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons. The isotopes of nickel, therefore, share the same atomic number because they are all nickel, but they have different mass numbers due to variations in the neutron count.
To answer the student's question, the correct option is: C. different mass, same atomic number.