Why is the nūn absent from «مُسْلِمُو» in «حَضَرَ مُسْلِمُو الْمَدِينَةِ»?

Noun Case Classes: Type 6 — Sound Masculine Plural and Related Forms

• Because it is a subject
• Because it became muḍāf
• Because it became indeclinable
• Because it is a broken plural