Write “the pens” in Spanish
There is no option to report this sentence. It will not accept "Las plumas". How can it be more flexible when there is no report option.
Go here : https://support.duolingo.com/hc/en-us/requests/new : in the drop down list make sure you change it to 'Report Bug'. And you tell them what language and what lesson it was in and what the question was. A screenshot might help them too.
Hope this helps!