The word unfastened is a verb. It is the past tense of unfasten.
adverb
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The buttons on my coat became unfastened.
Opened.
unfastened, unclosed
The part of speech for "answer" is a noun.
part of speech
The part of speech for this particular word is a noun.
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adverb
The part of speech for "explicit" is an adjective.
The part of speech for "indefinite" is an adjective.
The part of speech for "unfamiliar" is an adjective.