Which utility must be used to locate MPLS faults?

QoS (Quality of Service) is a network feature for managing traffic, not a utility for locating MPLS faults.

MPLS LSP ping (or `ping mpls`) verifies the reachability of an LSP endpoint but does not provide a hop-by-hop trace of the path to identify fault locations.

MPLS traceroute (or `traceroute mpls`) is a diagnostic utility that allows an administrator to trace the specific Label Switched Path (LSP) taken by packets, revealing each Label Switch Router (LSR) in the path and helping to pinpoint where a fault or forwarding issue might exist within the MPLS domain.

EEM (Embedded Event Manager) is a feature for event-driven automation and scripting on Cisco devices, not a direct utility for tracing MPLS paths or locating faults.