"Yours is a common question."
Translation:La tua è una domanda comune.
No expert here but as a statement of fact "La tua domanda è commune" ("Your question is common" ) seems valid to me.
I got that wrong too :( - guess it's not as accurate a translation as theirs.
I find it best when translating to stick as close as possible to the original sentence, not changing word order or getting too idiomatic. DL seems to be pretty literal-minded.
Even if the person is a man, "la tua" is for the "domanda". Domanda is a female word.