The correct option is: What is being painted by you?

Here's why the other options are not suitable:

• What were you painting? This uses the Past Continuous Passive Voice, which doesn't fit the present tense of the original sentence.
• What had been painted by you? This uses the Past Perfect Passive Voice, implying the painting is already finished, which contradicts the ongoing action in the original sentence.
• What was being painted by you? This option also uses the Past Continuous Passive Voice, making it inaccurate for expressing the present action.