Describe one measure of evolutionary fitness

The answer is letter A.

In the evolutionary sense fitness refers to the measure of reproductive success of an individual or of organisms in general. Those organisms that leave the largest number of mature offspring are the fittest. Fitness can be achieved through survival or mortality selection, mating success or sexual selection, and family size or fecundity selection. Fitness is critical for a species’ survival and is a major adaptation that all organisms do through a long period of time.  

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".