An unmanaged switch is the best solution because it is a plug-and-play device that does not require configuration, making it ideal for quickly expanding an office network without the need for advanced features. Unlike managed switches, unmanaged switches do not offer features such as VLANs or the ability to monitor network traffic, which aligns with the business owner's requirements. Routers and firewalls are not primarily used for expanding Ethernet ports and commonly serve different purposes, such as connecting networks and protecting them from unauthorized access, respectively. A Hub is not a good choice for a modern network due to its functionality of broadcasting all traffic on all physical ports.