Here's why:

* Ammonium nitrate (NH₄NO₃) is a salt formed by the reaction of a base (ammonia, NH₃) and an acid (nitric acid, HNO₃).

* In this reaction, the ammonium ion (NH₄⁺) is formed from ammonia (NH₃) by accepting a proton (H⁺).

* The nitrate ion (NO₃⁻) is formed from nitric acid (HNO₃) by losing a proton (H⁺).

Therefore, the parent acid of ammonium nitrate is nitric acid.