Final answer:
The most likely event that occurred in Pennsylvania in this scenario is the recession of a body of water, which would have exposed the land that later became a field where fossils were discovered.
Step-by-step explanation:
The event most likely to have occurred in Pennsylvania after these fossils formed, but before they were discovered in a field, is d. A body of water receded. This geographic change is indicated by the fossil record and is a common geological event. Such a recession would expose previous seabed areas, allowing for fossil formation and later discovery on land. This type of event has happened multiple times throughout Earth's history, including the drying up of the Delaware inland sea which formed extensive evaporite deposits and led to the formation of extensive caves and karst in the underlying reef system, as seen at Carlsbad Caverns National Park.