الحُسْنَى

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – الحُسْنَى is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

الحُسْنَى is the natural state of this definite noun and it remains الحُسْنَى even though it is in the genitive case due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

The final letter in this feminine nominal pattern الحُسْنَى is a weak letter (ي) weak letters do not show their endings, their endings have to be estimated (deduced by reason)

Good reward; Beauty

To be handsome, make good, seem good or beautiful, be excellent.

Surah/Below are the 3 verses in the holy Quran containing the genitive nounالحُسْنَى
Verse 
53-31
92-6and believes in the bestوَصَدَّقَ بِالْحُسْنَى
92-9and denies the bestوَكَذَّبَ بِالْحُسْنَى