Which verb form appears in mā ʿabadtum (109:4)?

Supplementary Practice 22: Comparative Rhetorical Study of Sūrat al-Kāfirūn and al-Naṣr

• ʿAbadtum is a plural imperative
• ʿAbadtum is a plural active participle
• ʿAbadtum is a jussive imperfect
• ʿAbadtum is a second-person plural past verb