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The answer to your question is the amount of neutrons so C.

The factor varies between the isotopes of an element neutron number

What is isotope?

A family of people often consists of related but not identical individuals. Elements have families as well, known as isotopes. Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons.

The number of protons in a nucleus determines the element’s atomic number on the Periodic Table.

For example, carbon has six protons and is atomic number 6. Carbon occurs naturally in three isotopes: carbon 12, which has 6 neutrons (plus 6 protons equals 12), carbon 13, which has 7 neutrons, and carbon 14, which has 8 neutrons. Every element has its own number of isotopes.

Thus the factor varies between the isotopes of an element neutron number

To know more about Isotopes follow

https://brainly.com/question/11680817