Explanation/Reference:
A: When a CE device is multihomed to two or more PE routers, the set of Ethernet links constitutes an Ethernet segment. An Ethernet segment appears as a link aggregation group (LAG) to the CE device .
B: When a CE device is multihomed to two or more PE routers, either one or all of the multihomed PE routers are used to reach the customer site depending on the multihoming mode of operation. The PE router that assumes the primary role for forwarding BUM traffic to the CE device is called the designated forwarder (DF).
Incorrect Answers:
C: An Ethernet segment must have a unique nonzero identifier, called the Ethernet segment identifier (ESI). The ESI is encoded as a 10 octet integer.
References: http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/evpn-bgp-multihoming- overview.html