"How are you doing?"
Translation:Quomodo te habes?
You want to think of "Quid agit?" as something more akin to "What's happening?"
Quid gets a sense of how, according to Lewis & Short, only when transferred into other phrases. Most of the examples seem to be surprise statements that are something akin to "huh?" or ambiguous in the English: "quid tum?" --> what then? how then?
OK, it seems that the developers probably intend to add audio to all the choices. In some cases choosing a word plays the audio. In some not. This exercise has no audio at all but others play audio for some words. I think it would be good to have audio on everything for auditory reinforcement. That's my vote!
It is not required because the verb ending tells you what the subject pronoun is, as in Italian and some other languages. I don't know if your answer is actually wrong, but in Italian it would be so unusual to put in the subject pronoun that it is often left out of the list of correct answers and so it gets marked wrong when it is actually technically correct. Maybe the same thing has happened with your answer. I think that so far I have learned to leave out the subject pronoun in Latin unless it is required for emphasis or to avoid ambiguity, as in Italian.