How is diversity among organisms explained according to evolutionary theory?

The correct choice highlights that all organisms share a common ancestry and are therefore united, but they exhibit diversity due to the processes of evolution. Evolutionary theory posits that over long periods of time, organisms adapt to their environments through mechanisms such as natural selection and genetic drift. These processes lead to variations among species, resulting in the vast array of life forms we see today. This shared ancestry and subsequent divergence is what accounts for the remarkable diversity, as organisms evolve different traits that help them survive and reproduce in varied ecological niches.

While other factors like environmental changes, geographical isolation, and genetic mutations certainly play important roles in the evolutionary process, the essence of evolutionary theory is the concept of a common origin from which diversity has evolved. This foundational idea encapsulates the relationship between all organisms, explaining both their similarities and the ways in which they have adapted and diversified over time.