"She is reading the text slowly."
Translation:Αυτή διαβάζει το κείμενο αργά.
Αυτή is optional in the sentence grammatically. Is it included in the correct answer so it explicitly state that the person reading is female?
It is included but may be used with or without. We can't know how much emphasis was meant. Many sentences are shown with the pronoun even when they are optional. Colloquial speech is without the pronoun. (unless we need it for emphasis, or course.)
Σιγά σιγά should be a way to write slowly, no? It certainly seems like one of the more common ways to say it.