Tafsir of Al-Qalam 68:40

Ask them: A change in the mode of address, directing it toward the Messenger of Allah, may Allah exalt him and grant him peace, by demoting them from the rank of being addressed directly. That is, ask them to rebuke them.

Which of them will guarantee that: A judgment external to the scope of intellects.

Guarantee [Za'im]: One who stands forth and undertakes to verify it. The interrogative sentence occupies the position of the second object of "ask." According to Abu Hayyan and a group [of grammarians], the verb is suspended from governing it due to the presence of the interrogation, and because the act of asking is placed in the position of knowing, as it is a cause for its attainment.