Eugene Has Been Given the Subnet 10

Eugene has been given the subnet 10.20.30.0/24 to use for the creation of a group of virtual machines to be used for the sales department. He is used to seeing a subnet mask in the format of four numbers separated by periods (255.255.255.0) along with the subnet ID, but isn't familiar with what the slash and a number mean after the subnet ID. He asks you to help him interpret the information. What term should you tell him to search for on a search engine to learn more about this topic?

A) CIDR
B) VPN
C) DDN
D) VLAN