Why is C correct and A is incorrect?

The statement in question distinguishes car alarms from burglar alarms. Unlike car alarms, burglar alarms actually are effective as deterrents. This is a reason that home alarms are good, or at least serve some redeeming purpose. A is incorrect because it is the opposite of what the sentence is doing. It does not justify restricting burglar alarms more than car alarms, it does the opposite. It is a reason why home alarms are actually effective, and why car alarms are ineffective. C is right because the statement tells us why we cannot simply ban home alarms, since they do in fact serve some purpose and are effective in deterring crime. The ban is the obvious alternative to the fine mentioned in C.