Answer:

In the concept of "survival of the fittest," the fittest organisms are those that are (a.)best adapted to their environment.

Explanation:

It means that they have genetic traits and characteristics that enable them to survive and reproduce in their specific ecological niche. The other options - being the largest, running the fastest, or producing the most offspring - may confer advantages in certain situations, but they are not necessarily the most important factors in determining an organism's overall fitness.