Plant material accumulated in these areas of indiana and eventually became the source of indiana’s abundant

Plant material accumulated in these areas of Indiana and eventually became the source of Indiana's abundant COAL DEPOSITS.
Fossils discovered in India indicated that during the Paleozoic era Indiana was a shallow sea and its marine life was abundant. The erosion from the distant Acadian mountain covered the sea and it became land where Indiana lies. Large amount of sand from the rivers were delivered to the delta and the plant materials accumulated on the deltas became the source of coal deposits. 

The right answer is c.sandstone.