* Atomic Number: The atomic number defines the number of protons in an atom's nucleus. This number determines the element.

* Stable Isotopes: Isotopes are atoms of the same element that have different numbers of neutrons. Stable isotopes are those that do not readily undergo radioactive decay.

* Looking at the Options:

* A) 56: This is the atomic number for Barium (Ba). Barium has several stable isotopes.

* B) 38: This is the atomic number for Strontium (Sr). Strontium also has stable isotopes.

* C) 100: This is the atomic number for Fermium (Fm). Fermium is a synthetic element, meaning it doesn't occur naturally. All isotopes of Fermium are radioactive and unstable.

* D) 20: This is the atomic number for Calcium (Ca). Calcium has several stable isotopes.

Therefore, the correct answer is C) 100. There are no known stable isotopes of Fermium.