What is the particular function of fāʾ in fa-huwa fī ʿīshatin rāḍiyah?

Supplementary Practice 26 — Comparative Rhetoric of Sūrat al-Qāriʿah and Sūrat al-Tīn

• It introduces the response to ammā
• It replaces the oath wāw
• It is an interrogative particle
• It governs the genitive of an iḍāfah