The correct answer is D. CX 8400. The requirement asks for a chassis solution, 100G speeds to the aggregation layer, and a maximum size of 10U. HPE Aruba Networking describes the CX 8400 as a core and aggregation switch with line-rate 10GbE, 25GbE, 40GbE, and 100GbE connectivity in a compact 8-slot chassis. The installation guide also identifies the CX 8400 as an eight-slot chassis platform for core and aggregation switching. This satisfies both the 100G requirement and the physical-size requirement because 8U is below the 10U maximum. The CX 10000, CX 9300, and CX 8360 are fixed-form-factor platforms rather than chassis solutions, so they do not match the customer’s chassis requirement. Therefore, the best initial proposal from the listed options is the HPE Aruba Networking CX 8400.