Final answer:
In scientific terms, a theory is a well-substantiated explanation of some aspect of the natural world. The theory of evolution, as a scientific theory, is best associated with the term 'fact,' which reflects its status as a widely accepted explanation based on substantial evidence.
Step-by-step explanation:
The term theory in the phrase 'theory of evolution' is often misunderstood. A scientific theory, such as the theory of evolution, is an explanation of an aspect of the natural world that has been substantiated through repeated experiments or testing.
In this context, 'theory' refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence and incorporates laws, hypotheses, and facts. Therefore, the most appropriate replacement for 'theory' in the term 'theory of evolution' would be fact (option a), as it is the closest match to the scientific meaning of a theory, which is an explanation that is well-substantiated and widely accepted within the scientific community.