Which of the following would best determine the fitness of an organism?

Explanation

In evolutionary biology, fitness is defined as the ability of an organism to survive and reproduce, thereby passing on its genes to future generations. The primary measure of fitness is the number of offspring an organism produces (Option A), as this directly indicates its reproductive success. Option B, focusing on health and lifespan, does not directly measure reproductive success. Option C, related to adaptability, does not directly correlate with the number of offspring. Option D, concerning the reproductive success of the organism's offspring, measures the fitness of the descendants rather than the organism itself.