D. Adjective; in the given phrase serene is an adjective part of speech.
More about part of speech:
Every word in the English language is referred to as a part of speech. Whichever part of speech a word belongs to is indicated by its function in a phrase. Words that fulfil several functions in a phrase are referred to as parts of speech. Some speech components can also serve the purposes of other speech components.
It has a number of eight parts of speech. Nouns, pronouns, verbs, adverbs, adjectives, prepositions, conjunctions, and interjections are examples of parts of speech.
Learn more about the part of speech here:
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