Which is the term that describes when the fossil record shows that two groups of organisms are related?

common ancestor.

Explanation:

when two groups of organism are related it means they share a common ancestry but may be they are not a direct parent but can relate to there far back generation.

Evidence from fossil's records are used to study that today how much organisms have changed themselves according to the environmental changes on earth. DNA and genetic codes mainly shows the shared common ancestry.

chert is a sedimentary rock composed of microcrystalline or cryptocrystalline silica. with this little bit of information, you can come to the conclusion that your answer is c.  

explanation:

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added