Wait...how does this fit into this lesson at all? The lesson is about objective pronouns, and there are none in this sentence at all. The only pronoun is the subject "det" ("it"). The object "anden" ("the duck") is not a pronoun.
If the question was "Er anden det?" ("Is the duck it?"), then it would fit the lesson. But as it is, it does not.
I think you are correct as I was confused too. I thought maybe this is "Er anden det?" but I somehow managed to answer this correctly
When you say somthing "is it a (insert noun) it does not matter what "it" is referring to or what the noun is. You always use det.
If you say "it is sweet" or any other adjective it becomes den or det depending on what "it" is referring to