Error (помилка у тесті)
I wrote: Ils écrivent leur lettres. And have such error:
Ils écrivent leurs lettres.
You used the plural "lettres" here, instead of the singular "lettre"."
It's better to report sentences during lessons or to discuss them in the French section where more people might see your question. Your problem here is that you used "leur" (singular) with "lettres" (plural). It should either be "leur lettre" (their letter) or "leurs lettres" (their letters). Does that answer your question?