Why does «الْمَسَاجِدِ» have kasra in «صَلَّيْتُ فِي الْمَسَاجِدِ»?

Noun Case Classes: Type 3 — Diptote (Ghayr Munṣarif)

• Because it is a subject
• Because it is genitive with the definite article «ال»
• Because it has tanwīn
• Because it is accusative