To be an index fossil, a fossil must
a. be present in rocks that are scatte.
over a large geographic area
b. contain remains of an organisms that
lived for a long period of geologic time
c. occur in small numbers within the rock
layers
d. have features that clearly distinguish it
from other fossils

a. They must be present in a large geographic area.

Explanation:

Index fossils can be used to determine the age of a rock strata with great precision, but they must be scattered over a large geographic area to be able to be used reliable for this type of measurement. Index fossils typically indicate a certain type of paleoenvironment belonging to a very specific prehistoric period and their formation coincides with the age of the rock layer.