The option that best describes what must have happened in the past in an area with a large petroleum trap is:
C. Organic matter must have been buried and placed under intense heat and pressure until it became petroleum. Then, the petroleum must have collected inside an impermeable underground rock formation.
This option accurately reflects the process of petroleum formation, which involves the burial of organic matter, its transformation into petroleum under heat and pressure, and subsequent accumulation in traps formed by impermeable rock layers that prevent the petroleum from migrating to the surface.