What do a mole of magnesium (mg) and a mole of iron (fe) have in common

they have the same number of atoms.

Explanation:

1) Mole is a basic unit of the SI (international system of measurements) used to measure the amount of a substance.

2) Mole is a fixed value and it is defined as the number of atoms in 12.000 grams of the isotope carbon - 12.

3) That approximate value of a mole is 6.022 × 10 ²³.

That means that both a mole of magnesium (Mg) and a mole of iron (Fe) (such as a mole of any element) have the same number of atoms which is about 6.022 × 10 ²³.

4) A more precise value for the mole is 6.0221367 × 10 ²³, but 6.022 × 10 ²³ is quite precise for most calculations in chemistry.

5) This number is known as Avogadro's number, in honor to the chemist (and lawyer) Amadeus Avogadro who in 1811 determined the number of a mole of gas.

the answer to your question is c