Homeostasis is best defined as:

Homeostasis refers to the physiological processes that organisms use to maintain internal stability despite external changes in their environment. This concept is crucial for survival as it enables living organisms to function optimally even when faced with varying temperatures, pH levels, or other environmental stressors. For example, human beings keep their body temperature around 37°C, regardless of external weather conditions, through mechanisms such as sweating or shivering.

The other choices describe different biological aspects but do not accurately capture the essence of homeostasis. The ability to reproduce pertains to a fundamental characteristic of life but does not relate to maintaining internal stability. The process of aging describes the changes that occur over time in living organisms, not their ability to regulate internal conditions. The formation of new species falls under the realm of evolution and speciation, which is distinct from the concept of homeostasis.