This question is very poorly constructed.
While c) is the only feasibly correct answer, you get there by process of elimination - a, b, and d are obviously wrong, leaving us with the vaguely worded c. It is not proper wording of the answer, IMO, since there is no such entity as an "illegal substitute", leaving you with the generic meaning of "the substitution was not legal."
One would expect, from the scenario offered, that the answer would have something to do with the proper ruling; instead to have the correct answer be, simply, that an unreported substitute is illegal is both confusing and overly generic.
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Tom
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