Hi... No, it is not ok. ¿Qué estan escribiendo ellos?, It is the most correct option
How to say .. what are you (plural) writing ? .. Is .. que escriben ustedes .. correct !! Is there a nother way ?
It may not be necessary in real conversion, especially when the subject has already been established. But Duo includes it to let us know (I think) that it wants us to use "they", since "escriben" can also refer to the plural "you".
Sometimes in Spanish they use present tense to indicate progressive. It's strange I know but it's not exactly like English. And in Questions they like to put the subject on the end to emphasize that it's a question, though it is optional.
The use of the present progressive (such as escribiendo), indicates that the subject of the sentence is performing the action right now, as we speak.
The use of the present indicative tense can indicate that the action is occurring right now, but it can also indicate a more general, ongoing activity.
It's a little easier for me to provide an example with the verb leer. If we change this to "¿Qué leen ellos?" it could translate as either "What do they read?" or "What are they reading?"
"What do they read?" would likely be asked to find out what kinds of material or what genre the subjects typically read. Do they read magazines, books, or newspapers? Or do they read mysteries? romance? biographies?
"What are they reading?" could be a question about a book club that has selected a particular book for this month, and someone wants to know which book. The people involved may or may not be reading right this second, but they are in the process of reading this month's selected book. The present progressive wouldn't be appropriate in this case. (Note that one could also ask "¿Cuál libro leen ellos?" to be more precise.)