This statement is best described as a fallacy of an ad hominem attack. Hence the correct option is c.
The statement is an example of an ad hominem attack, where the argument attacks the person making the claim rather than addressing the substance of the claim itself. In this case, the speaker dismisses Margaret's assertion that all blonds are brilliant by focusing on her hair color, implying that because Margaret's hair appears red, her judgment or credibility on the matter is questionable.
This form of argumentation is fallacious as it sidesteps the actual merit or evidence related to the claim about the intelligence of blond individuals and instead attacks the person presenting the idea. Effective arguments should address the content of the claim rather than attacking the character or personal attributes of the individual making the statement. Hence the correct option is c.