Why was modern classification invented?
a: traditional classification could not classify all living things.
b: scientists realized related organisms do not share similar characteristics.
c: scientists understood that species share a common ancestor.
d: modern classification is faster and easier to use.

The modern classification was invented because the scientists understood that species share a common ancestor. The correct answer will be C. 

I believe the answer is: C. Scientists understood that species share a common ancestor. 

Prior to modern classifications, Organisms are only divided into three kingdom, Bacteria, Archaea, and Eukarya

Because scientists understood that all of these organisms share a common ancestor, there is a need to categorize each organisms into a more smaller group using their specific characteristic as a basis in order to make research on each animals become easier/more focused.

Because of this, modern classification system was created.