What does the tanwīn do in «كَعَصْفٍ مَأْكُولٍ»?

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

• Assimilate into mīm with ghunnah
• Iqlāb before bāʾ
• Iẓhār before hamzah
• Idghām into lām without ghunnah