As currently conceived and implimented, a pure HUMS answers to your question as b and c, but certainly not a. there are systems that can help predict failure, but they need high software qualification standards, much input trending data, and lots of experience in the specific type of helo before they could be trusted. to my knowledge, and notwithstanding the assertions of some who have posted on pprune in the past, no current HUMS comes close to a reliable predictive capability.
None of this is to assert that such capabilities are not possible (they surely are). We must strive to make that predictive capability a reality.