Darwin's theory that the "fitter" survive and reproduce more offspring, which then inherit the traits that made their parents "fitter," is called a. mixed reproduction b. natural selection c. survival of the fittest d. heredity

The answer is B - Natural Selection. There exists selection pressures /forces on a population in a habitat. The population, on the other hand, have variations in genotype and phenotypes (traits) across individuals in the population. The selection pressures act upon these variations on a population to 'select' the superior traits that allow the population to best adapt to it's habitat. These traits are passed down generations by reproduction.

The higher you go up the less oxygen there is