Fossils and Geological Eras

Fossils are formed in a number of different ways, but most are formed when a plant or animal dies in a watery environment and is buried in mud and silt. Soft tissues quickly decompose leaving the hard bones or shells behind. Over time sediment builds over the top and hardens into rock.

Petrified fossils:- In geology, petrifaction or petrification is the process by which organic material becomes a fossil through the replacement of the original material and the filling of the original pore spaces with minerals.

Archaeospheroides is recognized as one of the oldest known microfossils, dating back to the Precambrian era, specifically associated with early blue-green algae.

The K-T boundary (Cretaceous-Tertiary boundary) marks the mass extinction event that occurred approximately 66 million years ago, separating the Cretaceous period from the Tertiary period.

The Himalayan and Alpine orogenies occurred primarily during the Cenozoic era, specifically starting in the early to middle parts of the era.

The earliest fossil evidence of land animals includes arthropods like centipedes and primitive spiders, which appeared during the Silurian or early Devonian periods.

The oldest fossil evidence of land animals, such as early arthropods, dates back to approximately 440 million years ago, during the Silurian period.

Dating is the general term for the process of determining the age of geological materials, including fossils and rocks, using methods like radiometric or relative dating.

From the given methods, the relatively most accurate method for dating of fossils is electron-spin resonance method. This method is based on the principle of measuring the number of charges occupied by the deep traps crystal band gap.

If fossil Y has vestigial structures that are homologous to functional structures in fossil X, it implies that X represents an earlier, more ancestral form, while Y represents a later form where the structure has become vestigial.

Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. Fossils document the existence of now-extinct species, showing that different organisms have lived on Earth during different periods of the planet's history. They can also help scientists reconstruct the evolutionary histories and ancestry of present-day species. For instance, some of the best-studied fossils are of the horse lineage. Using these fossils, scientists have been able to reconstruct a large, branching "family tree" for horses and their now-extinct relatives. 

Impressions are two-dimensional imprints of an organism that doesn't contain any organic material. An impression is a clue left of the organism's biological activity, they can be formed while the organism was moving or any other biological activity.
So the correct option is 'Impressions'.