what is it called when all of the organisms of a speicies completely die out. A= Mutations B= Adaptation C= Fossilization D= Extinction​

It is called Extinction. So D. Extinction

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.