I have the same question: What is the source? It looks more like an LSAT question than a GMAT, but I am not sure that it is structured correctly. I can find a justification for A as the answer, but the justification depends on interpreting the question in a way that might be too "aggressive". Here it is:

The stimulus says that a valid argument is one where the conclusion cannot be false if the premises are true. In other words, if the premises are true, so is the conclusion. What is ALSO true about a valid argument is something that the stimulus does not actually SAY: IF the premises are true, the conclusion can be EITHER true or false.

If all the premises of a circular argument are true, then the conclusion cannot be false, because it is one of the premises. So this fits the definition of a valid argument. But if the premises of a circular argument are false, then the conclusion CANNOT be true -- because it is one of the premises.

So the question is: Are we supposed to interpret the stimulus as saying that if the premises of a valid argument are false, then the conclusion must have the POSSIBILITY of being true? If so, then A is the correct answer. But that looks to me like a very big assumption rather than an interpretation.

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Grumpy

Kaplan Canada LSAT/GMAT/GRE teacher and tutor