Final answer:
In scientific terms, the word "theory" refers to a well-substantiated explanation, thus it is best replaced by "fact" in the phrase "theory of evolution" because it indicates an explanation supported by substantial evidence and research.
Step-by-step explanation:
The question involves the characterization of the word "theory" as it is used in the phrase "theory of evolution." In the context of science, a theory is a well-substantiated explanation of some aspect of the natural world that is based on a body of facts that have been repeatedly confirmed through observation and experiment. Thus, the word "theory" in the theory of evolution is best replaced by fact, as it represents an explanation that is well-supported by extensive evidence and scientific research. While the word hypothesis refers to a tentative explanation that can be tested and falsified, and idea and alternate explanation are more informal and less substantiated than a scientific theory, fact conveys the robustness and empirical support that the theory of evolution holds within the scientific community