Answer:
Over the mantelpiece
Explanation:
A prepositional phrase is a modifying phrase which includes a preposition (in this case, "over") and the object of the preposition (in this case, "the mantelpiece.")
Prepositions are words like "in, on, over, at, beside," etc, which then precede a noun. They are in "pre-position" in front of a noun, you might say (thus the term "preposition.") Long ago, the English language used to use inflected nouns (as a number of other languages do) to indicate things that we now accomplish through the use of prepositions. The time of transition between Old English (up to about AD 1150) and Middle English (from 12th to 15th centuries) is when prepositions became common in English usage.
