"Who is reading?"
Translation:Qui lit ?
This is a question about English actually. However, in English, you use simple present (you eat) when it is a constant or repetitive action (you eat cereals at breakfast), and continuous present (you are eating) when it is an action which is in progress at the present time (you are currently in the process of eating).
In French, there is no continuous verbal form. That is why you cannot directly translate the form : BE + verb-ING. You have to use a specific expression: "Être en train de + verb at infinitive.
So, "who is reading" = "qui lit ?" or "qui est en train de lire ?"