I think you are not thinking about the Table in the best way. Each plus sign indicates that when the mutant is "fed" the item indicated (ornithine, argininosuccinate, etc.) the mutant grows. The passage equates growth with the presence of arginine because it is necessary for growth. So, the last column is irrelevant. If we add arginine and that is what is needed for growth either mutant will obviously grow. However, the key is that when we added citruline the mutants DID NOT grow. That means the enzyme that converts citruline to argininosuccinate is likely defective. However, when we add argininosuccinate we DO get growth. That tells us the enzyme that creates arginine is likely functioning. The question asks for an enzyme NOT defective in both mutants, so C is best. Your other answer, B, must be false because the question asks for enzymes that are NOT defective in P or Q and the "no growth condition w/ citruline" suggests that argininosuccinate synthase is INDEED defective.
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Further Explanation on Question 3 of Chemical and Physical Sciences Section of GS-Free Practice Test
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