What is the purpose of the TTL field in the IP header?

used to mark routes as unreachable in updates sent to other routers
prevents regular update messages from reinstating a route that may have gone bad
prevents a router from advertisinga network through the interface from which the update came
limits the time or hops that a packet can traverse through the network before it should be discarded
defines a maximum metric value for each distance vector routing protocol by setting a maximum hop count

ccna exploration 2 chapter 4 Exam Question And Answers 

What is the purpose of the TTL field in the IP header?