* Diversity of life: The vast number of species (30 million estimated) directly demonstrates the incredible variety of life forms on Earth.

* Unity: While all life shares common ancestry, the sheer number of species highlights the differences and unique adaptations.

* Theory of evolution: The diversity of life is a result of evolution, but it's not the direct illustration of the theory itself.

* Natural selection: Natural selection is a mechanism of evolution that contributes to diversity, but again, it's not the direct illustration of the large number of species.

* e): There is no option 'e'.

In summary: The statement focuses on the sheer number of species, making the "diversity of life" the most fitting answer.