Translation:Mara? Are you listening?
-'a' is a verb suffix, turning a sentence into a yes/no question. The "Mara?" part of the exercise isn't a complete question, and doesn't have a verb, so you wouldn't add the interrogative -'a' to it.
Exercises like these would be clearer if they were ordered and punctuated like this:
Are you listening, Mara?
Mara, are you listening?
The Hint system for Duolingo seems to keep all the Hints entered for a word together and for each sentence it has an algorithm to choose which hint to place at the top and which two to also show. The algorithm doesn't seem to work as well with Klingon.
There are sentences where the Klingon name mara is placed in front of another noun to show that the thing mentioned belongs to Mara. Thus, at times the name mara should be translated into English as "Mara's". This is not one of those times, but didn't the Hint also show you two other possible translations?
Additionally, since you were doing a translation from Klingon to English, when you looked at the Klingon word mara, it was showing you some possible English translations. Since "mara'a" doesn't mean anything in English it wouldn't have shown you that. Actually, it doesn't mean anything in Klingon either. I wonder if you meant "mara'e'". If you were given the English sentence and asked for the hint, it might show you that mara'e' is one possibility in Klingon. However since this exercise was showing English translations for the Klingon word, it still would not have shown "mara'e'".