to remember what prepositional phrases are, just know that they are majorly location-based.
imagine a bee and a flower. the bee can fly over the flower, into the flower, under the flower, around the flower, etc., etc. these are all prepositional phrases. your bee was the subject and the flower was what it was acting towards.
The fountain in the garden is quite old.
in this specific sentence, the fountain is your subject. location? in the garden. your prepositional phrase would be: in the garden.
therefore, the words which make up the prepositional phrase are: in, the, garden.