"I think we are alone now."
Translation:Jeg tror vi er alene nå.
Tenke = ponder, have in mind
Tro = believe
Synes = opine
You can sit and "tenker raskt" (think quickly), "tenker på" or "gjennom" something, as in reflect on or think through something, and "tenker farlige tanker" ("think dangerous thoughts"), but it's not quite the right word with the practice sentence here.
Probably because of the "tenker". In this sentence, it doesn't make sense to use "tenker" rather than "tror", even though "tenker" is the literal translation. There's just something about it that makes it off, and I wish I knew how to explain why. Perhaps the course contributers can explain it better, but from my standpoint I can just see why it failed you.