Which path attribute refers to the external metric of a route?

a. Lowest multi-exit discriminator
b. Lowest origin type
c. Multiple paths
d. External paths

a. Lowest multi-exit discriminator
Explanation: Lowest multi-exit discriminator
(MED): This is external metric of a route. The metric with a lower MED value is preferred over a higher value. The MED value is another widely used attribute to manipulate route selection. Unlike local preference that influences the routing decision within the local AS, the MED value is used to influence the routing decision of an adjacent AS.
Lowest origin type: The origin attribute that defines the origin of the path. If the path is originated from protocols such as Interior Gateway Protocol (IGP), it has lower preference to the Exterior Gateway Protocol (EGP).
Multiple paths: This is used to determine if multiple paths require installation in the routing table.
External paths: If two paths are external, BGP prefers the path that was received first.