Which of the following is not an example of a fossil?
a. a hardened footprint left by an animal
b. an insect trapped in amber
c. a petrified plant
d. an ancient rock found in a dry riverbed

Option (D)

Explanation:

Fossils are defined as the dead decay of ancient plants and animals that are buried millions of years back. These fossils are found mostly in the sedimentary rock types because of the presence of softer sediments like silt and clay that undergoes compaction and lithification.  

In order to become a fossil, the following conditions must be fulfilled-

1) Hard parts of the organisms must be present

2) Immediate burial must take place

3) Detachment of the body from the oxygen-rich environment

Some examples of fossils are shells of brachiopods, cephalopods, and gastropods. It also includes ancient insects trapped in amber, petrified wood and trace fossils like footprints and burrows.

Thus, the correct answer is option (D).