In the scenario described, the most significant issue with the core switch selection, according to Aruba Campus Access learning resources, is answer A: "The core switch will not support the 25GbE downlinks to the distribution switches." This is a critical consideration because the bandwidth capabilities between the core and distribution layers significantly impact the overall network performance and scalability. If the core switch cannot support 25GbE downlinks, it may create a bottleneck, preventing the distribution switches from operating at their full capacity and affecting the performance of connected devices and applications. Ensuring the core switch has the necessary port speeds and densities to support the intended design and traffic patterns is crucial in network design, as emphasized in Aruba's documentation on campus network architectures.