Question

The most common fossils are of:

vertebrates
invertebrates
bacteria
plants

I'm guessing it is vertebrates.

Answer 1

D.

The problem with animals, including bacteria, is the live tissue which rots and decomposes through the action of mold and other natural processes in nature, whereas plant matter is relatively long lived without rotting, long enough to become covered in silt and to to eventually become a fossil. Silting over and then turning into a fossil requires a LOT of time, and vegetable matter is the only thing to last long enough to be in the majority.

Answer 2

The answer to your question is Molluscs and brachiopods include some of the most common fossils like clams, sea shells and belemnites and also some of the most beautiful like sea-snails, nautilus and ammonites. Generally they are the remains of marine creatures and so are only found here in Jurassic and Cretaceous aged rocks.
Hoped I Helped!