Which analysis explains the genitive bayḍati?

Imruʾ al-Qays: A Guarded Tent, the Pleiades, Objection, and Dunes (Verses 23–29)

• Treating the first letter as an accusative particle
• Declaring bayḍah indeclinable in every context
• Treating tamattaʿtu as a preposition
• An understood rubba after the wāw