* Binary fission is a simple, asexual method of reproduction where a single prokaryotic cell divides into two identical daughter cells. This process is very efficient and allows for rapid population growth.

Let's look at the other options:

* Fight disease: Prokaryotes don't have the same immune systems as multicellular organisms. They can be infected by viruses (bacteriophages) but don't fight disease in the same way.

* Vary their shapes: While some prokaryotes have different shapes, binary fission doesn't contribute to this variation.

* Adapt easily to any environment: Prokaryotes can be very adaptable, but binary fission is a mechanism for reproduction, not adaptation. Adaptation happens through mutations and natural selection.

In summary: Binary fission is a significant advantage for prokaryotes because it allows them to reproduce quickly, increasing their population size rapidly in favorable conditions.