What advantage of placing the IS-IS layer 2 flooding domain boundary at the core Layer in a three- layer hierarchical network is true?

Level 1 and Level 2 domains are by definition separate in IS-IS; overlapping them is not an advantage of this design choice.

Placing the boundary at the core does not simplify Level 1 domains - Level 1 complexity is determined by the number of routers and links within each area, independent of where the Level 2 boundary sits.

This boundary placement is specifically suited to hierarchical three-layer topologies and is not universally applicable to any kind of topology.

When the Level 2 domain boundary is placed at the core, Level 2 LSPs are confined to core routers only, reducing the size of the Level 2 LSDB and preventing churn from access or distribution topology changes from propagating into the backbone. This containment makes the Level 2 domain smaller, faster to converge, and more stable overall.