The old combine on the farm had not been operating in almost a decade, but after a few hours of work, it was operating flawlessly in the field.
on the farm -- prep phrase (adjective) in almost a decade -- prep phrase (adverb) after a few hours of work -- prep phrase (adverb) in the field -- prep phrase (adverb)
So there are 4 prepositional phrases in this sentence.
E) Central Idea. The statement given here is what the idea for writing the essay would be, making it the central idea of the essay. Hope this helps! :D