Why is the adjective in اَلنَّفْسُ الْمُطْمَئِنَّةُ feminine and definite with al-?

Practical Exercises with Adjectival Phrases

• Because the noun اَلنَّفْسُ is definite and feminine by established usage
• Because the noun described is indefinite
• Because نَفْسٌ is a masculine noun