The /23 CIDR notation is the correct answer because it offers a greater number of subnets compared to a /22 subnet while still providing a substantial number of host addresses. A /23 allocation gives 512 addresses in total, with 510 usable host addresses after reserving for network and broadcast addresses, making it suitable for medium-sized departments. In contrast, the /25 CIDR has fewer host addresses which might not be enough for most departments, and the /21 CIDR has too many host addresses which could lead to inefficient use of IP space if multiple subnets are needed. Furthermore, /20 and /30 CIDR notations either provide an excessive number of host addresses or not enough subnetting flexibility.