Final answer:
The group of words in bold is a phrase that provides additional information about the hippopotamus.
Step-by-step explanation:
The group of words in bold, 'an extremely dangerous and highly aggressive animal', is a phrase. A phrase is a group of words that does not contain a subject and a verb, and it does not express a complete thought. In this sentence, the phrase is used to provide additional information about the hippopotamus, describing its nature.
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