All atoms, isotopes, and ions of Magnesium have the same... atomic number
mass number
all of the above
charge

option A: Atomic number

Explanation:

The atoms, isotopes and ions of magnesium doesn't change the atomic number of the magnesium.

For example, Isotopes are atoms that have the same number of protons but at the same time have different numbers of neutrons. This means that atomic number remains the same while mass number differs. Now, an ion is defined as an atom or molecule that has either a positive or negative charge.

So only option A is correct: Atomic number

there are two significant figures.