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 used to identify and date the layers of rock in which they are found. They are typically extinct organisms (this helps in correlating rock layers across different locations). Organisms with soft parts are less likely to form good - preserved fossils (hard parts like shells, bones are more likely to fossilize). Index fossils are not generally large (many are small, like certain types of microfossils). And they lived in a wide geographic area (so that they can be used for correlation over large regions), not a narrow one.

Answer:

are extinct