Which command displays approximate travel times and discovers the maximum transition unit along the path?

The command that displays approximate travel times and discovers the maximum transmission unit (MTU) along the path is tracepath. This command is particularly useful because it not only shows the route packets take to reach their destination but also provides information on the MTU of each hop along that path. This can help network administrators diagnose potential issues related to packet fragmentation and overall network performance.

While other commands like ping and traceroute provide valuable information regarding reachability and network path tracking, they do not combine the ability to determine MTU along with the latency measurements as effectively as tracepath. Ping focuses primarily on checking the reachability of a host and measuring round-trip time without revealing the MTU specifics. Traceroute traces the path packets take to a destination but doesn't give you the MTU size for each hop explicitly. The command iftop is used to monitor bandwidth usage on interfaces, which is unrelated to path discovery or MTU measurement.

Thus, tracepath stands out as the tool that combines both latency information and MTU discovery along the path to a network destination.