@stefgonz bingo! The conclusion is "people cannot feel secure" not "people might not feel secure."
This is a Strengthen with Sufficient Premise question, so we are looking for the answer choice that 100% guarantees the conclusion.
The conclusion here is, "Thus, under vague laws people cannot feel secure."
The support provided?
"Vague laws set vague limits on people's freedom, which make it impossible for them to know for certain whether their actions are legal."
Notice the jump here.
Vague laws make it impossible for people to know for certain whether their actions are legal but the conclusion jumps to people cannot feel secure.
(A), "people can feel secure only if they know for certain whether their actions are legal," guarantees this conclusion by closing this gap (i.e. by making not knowing for certain whether actions are legal sufficient for not feeling secure).
FS ==> KCWAL not KCWAL ==> not FS
Hope this helps! Please let us know if you have any additional questions.
