Final answer:
Circular reasoning occurs when the conclusion of an argument is used as a premise to support itself.
Step-by-step explanation:
A clear example of circular reasoning is option D: The soundness of the theory is proven by how sound the theory is. Circular reasoning occurs when the conclusion of an argument is used as a premise to support itself. In this example, the statement is essentially saying that the theory is sound because it is sound, which is a circular and illogical argument.
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