Question

which phrase describes organisms that formed index fossils?
are extinct
had soft parts
were generally large
lived in a narrow geographic area

Explanation:

Index fossils are fossils of organisms that are used to define and identify geologic periods. They must be extinct (so they don't continue to exist in younger rock layers and confuse the age - determination process). Organisms with soft parts are less likely to form fossils (hard parts like bones or shells fossilize more easily). Index fossils are not generally large (smaller organisms can also be widespread and have short - lived species which are characteristics of index fossils). Index fossils lived in a wide geographic area (so they can be used to correlate rock layers over large regions), not a narrow one.

Answer:

are extinct