The selected IS-IS design must place the correct routers as Level 1/Level 2 boundary routers so that area failures stay local while traffic can still reach backbone application services. In a hierarchical IS-IS design, Level 1 routers exchange detailed topology only inside their own area. Level 2 routers form the backbone used for interarea connectivity. L1/L2 routers connect the local area to the backbone and advertise reachability between the two levels. Areas 20 and 30 are expected to double in size, and link flaps inside those areas must not impact the backbone. Therefore, the design should keep internal area routers at Level 1 and use only the boundary devices that connect those areas to the backbone as L1/L2. Option A matches that principle by selecting the appropriate boundary routers for interlevel connectivity while avoiding unnecessary L1/L2 adjacencies throughout the topology. The other options either place the wrong routers in Level 2 only or fail to provide a clean transition between local areas and the backbone. Reference topics: IS-IS hierarchy, L1/L2 boundary design, area containment, backbone stability, route leaking.