Wrong flaw. Like the stimulus, this erroneously links two terms (”fur” and “not a fish”) just because they are necessary conditions to the same sufficient condition (”cat”). Unlike the stimulus, however, (A) negates one of the terms: in the conclusion, the terms are “fish” and “fur,” but in the premises, the terms are “not a fish” and “fur.” Because that negation doesn’t occur in the stimulus, this isn’t the correct answer.

Wrong flaw. Like the stimulus, this erroneously links two terms (”fur” and “not a cat”) just because they are necessary conditions to the same sufficient condition (”dog”). Note that the argument would be valid if the conclusion were “some things with fur aren’t cats,” but this argument negates the terms to reach the invalid conclusion “some cats lack fur.” This isn’t the same flaw we see in the stimulus, though, where the erroneous link between the necessary conditions isn’t in the form of a “some” relationship.

As shown below, this stimulus sets up two conditional relationships, each with the same sufficient condition:

If it’s a cat, it has fur.
If it’s a cat, it isn’t a dog.

Because cats both have fur and aren’t dogs, the author concludes that nothing with fur is a dog. This is the same flaw and structure from the stimulus, where the argument assumes a conditional relationship between two terms just because they share a sufficient condition.

Wrong flaw. Like the stimulus, this erroneously links two terms (”(not a) cat” and “(not a) fish”) just because they are necessary conditions to the same sufficient condition (”dog”). Unlike the stimulus, however, (D) negates both of the terms in the conclusion: in the premises, we are dealing with “not a cat” and “not a fish,” and in the conclusion, we end up with “cat” and “fish.” Because that negation doesn’t occur in the stimulus, this isn’t the correct answer.

No flaw. If it’s a dog, it’s a mammal; if it’s a mammal, it’s not a fish. By following this chain of conditional claims, we can validly conclude that if something is a dog, it’s not a fish—in other words, “no fish is a dog.” This is a valid argument, so it can’t be the correct answer!