The part of the sentence is a prepositional phrase is over the mantelpiece. Thus 3rd option is correct.
A prepositional phrase is a collection of words that includes a preposition, a noun or pronoun as the preposition's object, and any object modifiers. In this kind of the phrase preposition comes first than the object.
In the above sentence, over the mantelpiece is the prepositional phrase as it contains preposition and noun in the sentence. In the sentence preposition is over and mantelpiece is the noun.
Thus 3rd option is correct.
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