Here's why:

* Iron pyrite is a mineral composed of iron and sulfur.

* Iron(II) indicates that iron has a +2 charge (Fe²⁺).

* Sulfide refers to sulfur with a -2 charge (S²⁻).

* To balance the charges, we need two sulfide ions for every one iron ion.

Therefore, the correct formula is FeS₂.

Important Note: While iron pyrite is often called "fool's gold" because of its golden color, it does not contain any gold.