What spelling change occurs in بِنْتَاهُ, 'his two daughters,' when an attached pronoun joins the dual muḍāf?

Comprehensive Translation and Sentence-Building Practice

• The final nūn of بِنْتَانِ is dropped, and the pronoun attaches to the alif
• It becomes بَنَاتٌ
• It becomes بِنْتُهُ