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It is a convoluted argument. I don't know how to get the answer: why "justify an action" is equal to "justify a means"?

I think you're making the argument more complicated than it should be. The first sentence indicates that the end value is based on intent, so if you pair that with the conclusion that nothing but an end's value can justify your "means", or reason for action, then answer C becomes the clear paraphrase to that argument.