A vs C

Sorry I realized my question may be unclear, here is a clearer question: I chose C over A as the answer because there could be many other reasons why one does not feel secure. Answer A says that the only way to feel secure is through knowing which actions are legal. Why is this thinking wrong?

We are trying to find an answer choice that, if true, makes the argument correct. It seems like you're treating this as a must be true, which this is not. A makes the argument correct. The author wrongly thinks that vagueness means people cannot be secure because they can't know what's legal. However, we have no reason to think that not knowing what's legal means one cannot be secure. A fixed this flaw, it bridges the gap. C is the converse. It tells us that if secure then know what's legal. The author is missing a premise saying that if know what's legal, then feel secure.