The fossil record is incomplete and does not contain all of the Earth's past organisms. Why have some organisms been found as fossils while others have not? A. Fossils only exist for organisms that lived over 3 million years ago.
B. Fossils only exist for organisms that had shells or hard outer body coverings.
C. Only very large organisms have enough body mass to result in fossil formation.
D. Most organisms die and decompose without leaving any fossil remains.

The answer is D.

Explanation:

We can easily knock out answer choice A, as fossils only take around 10,000 years to form completely. Not just for the organisms that lived 3 million years ago. Answer B as well, since theres been fossils for about just every animal out there. Answer C, we have seen the tiniest of organisms turn into fossils before, so this is outright wrong. Answer D, is correct. Why? Because fossils need the PERFECT conditions to be properly made. We´ve all seen a dead cat on the road or two. Everytime we visit, we notice how slowly more and more deterioated the body gets. Until theres nothing but maybe the skull and some other few spare bones left. Which those bones, soon turn into fertilizer for the ground / dirt beneath it.