In The Republic, Plato discusses justice and virtues within the context of a well-ordered society. Moderation, or self-control, is considered one of the key virtues and is not confined to a specific group but is present throughout the entire republic. Plato argues that moderation exists when each class—guardians, auxiliaries, and money makers—conforms to its appropriate role and desires no more than what is necessary for its function. Thus, moderation is present in all parts of the society.
Option (A) is incorrect because moderation is not limited to the guardians, though they do play a crucial role in upholding it.
Option (B) is partially correct, as the guardians and auxiliaries embody moderation in their duties, but moderation is not restricted to them alone.
Option (C) is incorrect because the money makers, according to Plato, are focused on wealth accumulation and not necessarily embodying moderation in the same way as other classes.
Option (D) is correct because Plato argues that moderation is a virtue that should be present in all parts of society to achieve harmony.
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\boxed{\text{The correct answer is (D).}}
\]