Why is “الدَّرْسُ” the musnad ilayh in “كُتِبَ الدَّرْسُ”?

ʿIlm al-Maʿānī: Khabar, Inshāʾ and the Purposes of Reporting

• Because it is the passive subject (nāʾib fāʿil)
• Because it is an adverbial
• Because it is a preposition
• Because it is an imperative