Change the voice: We are warning you for the last time.

You are warned by us for the last time.

You are being warned by us for the last time.

You had been warned by us for the last time.

None of these

Tense: Present continuous ("are warning")

Passive voice structure: am/is/are + being + past participle

"are warning" → "are being warned"

Correct passive voice: You are being warned by us for the last time.