Everything has been building to this question. Arjuna has seen the devas enter. He has watched warriors crushed between teeth. He has described rivers, moths, fire. And now, shaking, prostrating, he asks the only question that matters: 'Who are You?'
Notice what Arjuna adds: 'I do not understand Your purpose.' He knows this is Krishna. He has no doubt about the identity. But the intent behind this devastating form — the reason for all this destruction — that is what he cannot grasp. It is one thing to see a fire. It is another to understand why it burns.
The address 'Devavara' — greatest of the devas — and the plea 'Prasida' — be gracious — tell us that Arjuna is not demanding an answer. He is asking from his knees. This is the posture of a devotee who has reached the limit of his own understanding and has no recourse but the grace of the one before him.