Explanation
A core switch is the most likely device causing the issue where users on each floor have stable connectivity, but do not have connectivity to other floors. A core switch is a high-performance switch that connects multiple access switches in a network. An access switch is a switch that connects end devices, such as computers and printers, to the network. A core switch acts as the backbone of the network, providing interconnection and routing between different subnets or VLANs. If the core switch is malfunctioning or misconfigured, it can prevent communication between different segments of the network, resulting in intermittent connectivity issues. References: [CompTIA Network+ Certification Exam Objectives], Core Switch vs Access Switch:
What Are the Differences?

To get 500 hosts you need 9 host bits, thats gives us an mask of 255.255.254.0 /23. This gives us 510 hosts, with 508 useable addresses. They did not mention expandability, so we should not read that into the answer, they did however, asked to have the fewest unused addresses.