The correct blank is Fossil.
In order to form fossil, the body of the organism after death should not be eaten by the scavengers of the environment.
The body of the organism must not be eaten or destroyed and should be preserved in order to form a fossil.
It takes a long time for the formation of the fossil, if the body remains untouched for several years. The body of the organism is buried and compaction takes place. This creates impression of the animal and formation of fossil takes place.