Question

these specialized fossils indicate to geologists the boundaries in geological time and determine the age of rocks. limestone fossil amber fossil cast fossil index fossil

Explanation:

Index fossils are fossils used to define and identify geologic periods. They are abundant, widely distributed, and existed for a short period of time. This makes them useful for indicating geological time boundaries and determining the age of rocks. Limestone is a rock type, not a fossil type for this specific purpose. Amber fossils are preserved in tree resin and are more about specific organism preservation. Cast fossils are a type of fossil formation (a replica of an organism), but not specialized for geological time - boundary indication like index fossils.

Answer:

Index Fossil