If you think libertarianism argues that a) is the correct and proper action then you don't understand libertarianism. I'm not even sure how you'd arrive at the idea that it does. I'm guessing that you are trying to make some kind of analogy between libertarian attitudes to government and libertarian attitudes to individual interactions but that you are assuming ideas about government that libertarians do not share.

As for the natural disaster scenario, the basis of libertarian ethics is that people should not be compelled to do anything by force. Voluntary charity is perfectly compatible with libertarianism and indeed libertarians often believe that voluntary charity is a much more satisfactory solution to most of the social problems that governments currently take it upon themselves to address.