What is difference between het and de?
"de" is used for masculine and feminine nouns, "het" is used for neuter nouns. See here for more info on the Dutch articles.
Here is a list of all the grammar explanations that have been posted so far, for future reference.
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Why "het dier" ??
To make things more interesting, exercises like this should always include both"het" and "de" as possible answers.