Natural selection is considered a theory in biology primarily because it is a comprehensive explanation of how evolutionary processes occur, and it is supported by extensive evidence gathered from various scientific fields, including genetics, paleontology, and ecology. Among the provided responses, the best answer would be:
It involves the formation of all living things on the planet.
While this statement may be a bit broad, it aligns with the concept that natural selection is a foundational principle in understanding biological diversity and the evolution of life on Earth. However, it’s important to note that the strength of natural selection as a theory comes from its extensive supporting evidence, so "There is lots of evidence to support it" is also a significant aspect of why it is classified as a theory.