From which derived form is the imperative arkhī?

Imruʾ al-Qays: Dialogue in ʿUnayzah’s Litter, Boasting, and an Oath (Verses 13–18)

• Form V, tarakhkhā yatarakhkhā
• Form III, rākhā yurākhī
• Form IV, arkhā yurkhī
• Form I, rakhiya yarkhā