Which letter follows the sākin mīm in «عَلَيْهِمْ طَيْرًا»?

Al-Fīl and Quraysh: Applied Iẓhār, Ikhfāʾ, Idghām, Madd and Waqf

• Wāw, therefore nūn assimilation
• Ṭāʾ, therefore clear mīm
• Bāʾ, therefore concealment
• Mīm, therefore assimilation