Explanation/Reference:
D: When customer networks that use private addresses connect to the Internet infrastructure, the private addresses might overlap with the same private addresses used by other network users. MPLS/BGP VPNs solve this problem by adding a route distinguisher.
A: The PE router uses the route target to constrain the import of remote routes into its VRF tables.
Incorrect Answers:
B: Multiprotocol BGP is not required for L3VPNs.
Note: Multiprotocol Extensions for BGP (MBGP), sometimes referred to as Multiprotocol BGP or Multicast BGP and defined in IETF RFC 4760, is an extension to Border Gateway Protocol (BGP) that allows different types of addresses (known as address families) to be distributed in parallel.
In Junos OS, Layer 3 VPNs are based on RFC 4364. RFC 4364 VPNs are also known as BGP/MPLS VPNs because BGP is used to distribute VPN routing information across the provider's backbone, and MPLS is used to forward VPN traffic across the backbone to remote VPN sites.
References: http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/layer-3-vpn- overview.html