What is route poisoning?

What is route poisoning?

1. a) It sends back the protocol received from a router as a poison pill, which stops the regular updates.
2. b) It is information received from a router that can't be sent back to the originating router.
3. c) It prevents regular update messages from reinstalling a route that has just come up.
4. d) It describes when a router sets the metric for a downed link to infinity.