Variation refers to the differences that exist between individuals of the same species. In humans, these differences are evident in unique genetic markers, such as DNA finger prints and blood groups. Because offspring inherit half of their DNA from each biological parent, scientists can analyze these specific variations, to confirm biological relationships with high accuracy. Therefore, identifying unique genetic variations is a direct application used to establish paternity.

B. sex determination is a biological process determined by chromosomes (X and Y) at the moment of conception, rather than an "application" of the observable variations found within a population.

C. population control refers to the practice of artificially altering the rate of growth of a human or animal population, which is a management strategy rather than an application of biological variation.

D. survival of the fittest is a consequence or mechanism of natural selection, where variation allows certain individuals to adapt better to their environment. While variation makes it possible, "survival of the fittest" is a theory, not a practical human application like paternity testing.