"The old people are eating cookies."
Translation:Starzy ludzie jedzą ciasteczka.
That moment when it'sa multiple choice question, and Duo gives you the option 'Starzy ludzie jedzą dzieci'
The English sentence lacks hints for the correct verb tense in the multipart hints (which show up first), which ends up that burying the correct verb tense to third place and in a split screen - someone who's still not acquainted with the endings may stumble and pick "jesz" or "jecie" instead. I've already reported it :)
"Ciasteczka" typically refers to "little cookies" in Polish, whereas "ciastka" are often referred to as regular cookies.
A cake is ciasto (plural ciasta), a large cookie is ciastko (ciastka), and a small cookie is ciasteczko (ciasteczka).
There's no "ci" in the main translation, although is is accepted.
"ci" = "these" for the masculine-personal plural. "ludzie" include at least one man (by definition), so the word is masculine personal.
Often when in English you use "the", in fact the meaning is more like "this"/"these". So this is why it's accepted. Because sometimes it just makes more sense than the literal translation.