"It is a duck."
Translation:To jest kaczka.
Exactly, you could point with your finger at something and say: "To jest kaczka." = "This is a duck.", "To jest pies." = "This is a dog.", "To jest dziecko." = "This is a child." ("kaczka" is feminine, "pies" is masculine, "dziecko" is neuter).
But you would say: "Ta kaczka jest czerwona." = "This duck is red.", "Ten pies jest czerwony." = "This dog is red.", "To dziecko jest czerwone." = "This child is red." (note that now you use "ta/ten/to" which indicates the gender; note also how the endings of the adjective differs in each sentence: "-a/-y/-e").
"Jest kaczką" on its own is a pronoun-less version of "on/ona/ono jest kaczką" and I think that only 'ono' works here if you don't want to insult anyone. However 'ono' is very rare and practically always replaced by 'to' if you want to say "it is a...".
And the presence of 'to' changes the case requirements to nominative. So we really only accept "To [jest/] kaczka" in this exercise.
"Jest kaczka" is technically correct, but that means something like "there is a duck" or "we have duck".
It is pretty unusual and I don't think it works here. Generally a sentence built as "[This/That/It] is a noun" should start with "To".
The only situation I can imagine in which "Jest kaczką" works, is in a dialogue like:
- So, Mickey is a mouse and Goofy is a dog... and what about Donald?
- He's a duck.
That last part could be rendered by "Jest kaczką". But outside of a conversation like that, it doesn't work. You can't point at a duck and say "Jest kaczką".
Also, actually "Donald jest kaczorem". "kaczor" is a male duck ;)