Which term is defined as "the basic unit of structure and function found in all living things"? tissue, organ, organelle, or cell?

About which of these did darwin have a poor understanding? a) that much of the variation between individuals in a population is inherited b) the factors that cause individuals in populations to struggle for survival c) the sources of genetic variations among individuals d) that individuals in a population exhibit a good deal of variation e) how a beneficial trait becomes more common in a population over the course of generations