Index fossils are used to determine the relative ages of rock and fossils and are also used to
A) define the boundaries between geologic periods
B) determine the absolute age of surrounding rock
C) identify the fossil species found in rock layers
D) infer evolutionary relationships between organisms

Index fossils are used to determine the relative ages of rock and fossils and are also used to define the boundaries between geologic periods.

Option: A

Explanation:

The fossils which are recognized as fossils guides or indicator fossils are used to classify and recognize geological or faunal periods, termed as index fossils. It must be of short vertical reach, wide geographic distribution and swift patterns in evolution. It helps to assess the rock layers ' age and helps to date other fossils found close and around them. For an instance, Ammonites were abundant in the Mesozoic period between 245 to 65 mya, they have not been found after the Cretaceous era, as they became endangered during the K-T extinction (65 mya).