What is the grammatical function of al-wusṭā?

Iṭnāb: Clarification after Ambiguity, Tawshīʿ, Repetition and Īghāl

• An independent noun coordinated with al-ṣalāh
• A complete substitute for al-ṣalawāt
• An adjective qualifying al-ṣalāh
• An absolute object of ḥāfiẓū