Comprehensive and Detailed Explanation:

Inter-AS MPLS L3VPN solutions enable communication between differentAutonomous Systems (AS)while maintaining VPN separation. The three primary solutions for inter-AS MPLS L3VPN are:

Option A (Back-to-Back VRF)

MPLS isnot requiredbetween ASBRs.

VPN routes are exchangedwithout label switching.

Each ASBR has aVRF instanceconfigured and forwards traffic usingplain IP routing.

Option B (eBGP Redistribution of Labeled VPNv4 Routes)

MPLSis not requiredbetween ASBRs.

ASBRsexchange labeled VPNv4 routes using eBGP.

The ASBR doesnot perform label switchingbut simply forwards packets based on VPNv4 labels.

Option C (Multi-Hop MP-BGP for VPNv4 Routes)

MPLS is requiredbetween ASBRs.

ASBRs establishlabel-switched paths (LSPs)to forward VPN traffic across AS boundaries.

SinceOption Brelies onlabeled VPNv4 routes via eBGPand does not need MPLS between ASBRs, the correct answer isD.

????Reference:Huawei HCIE Datacom Study Guide - MPLS VPN Inter-AS Solutions