Answer: The italicized word ("trained") is a participle.
Explanation: A participle is a non-finite verb form. There are two types of participles: present participles and past participles: While present participles generally function as nouns, past participles usually act as adjectives. In the sentence provided, the italicized word, which is "trained", is a past participle. "Trained" is functioning as an adjective since it is describing and providing additional information about a noun: "dog".
