Right answer is sometimes not available on the multiple choice sentences!
Sometimes I pick my best guess and strike out on the question because the correct answer isn't even available on the multiple choice questions?!
If Duolingo makes a mistake like that, try clicking the Support tab on the left. You can email your problem, and if it is their mistake, then they'll fix it. Otherwise, it's your mistake-- maybe you forgot the answer or read it wrong.
I am having the same problem, at this point in the present perfect sentences. The correct answer isn't one of the choices, although the numbering of the answers jumps from 3 to 5.