Recall the Framework: The Marketing Mix is a fixed set of 4 Ps — Product, Price, Place, Promotion — worth memorizing as a group.
Since the question gives three already (Product, Price, Place), the missing fourth by definition is Promotion — the communication/advertising piece of the mix.
People, Process, and Profit belong to OTHER marketing frameworks (like the extended 7Ps for services), not the classic 4Ps being asked about here.