Which of the following best describes a scientific theory?

A scientific theory is fundamentally an explanation that is grounded in evidence and can evolve with the introduction of new findings. The strength of a scientific theory lies in its ability to unify a broad range of observations and experimental results under a cohesive framework. As new evidence is discovered through research and experimentation, theories can be refined, expanded, or sometimes even overturned, reflecting the self-correcting nature of scientific inquiry. This adaptability is what distinguishes a scientific theory from a mere hypothesis or fact, as it remains open to modification based on ongoing scientific investigation.

In contrast, other options fail to encapsulate the dynamic nature of scientific theories. A conclusion derived from observation, while important to scientific reasoning, describes results rather than a comprehensive explanatory model. An unmodifiable fact suggests a rigidity that goes against the essence of scientific exploration, where even facts are subject to reevaluation with new evidence. A simplified model of complex systems, while useful in certain contexts, does not encompass the broad explanatory power and evidential backing that a scientific theory provides. Thus, the option highlighting a scientific theory's capacity to change with new evidence best captures its essence.