Which command would be used to enter VLAN configuration mode on a switch?

To enter VLAN configuration mode on a switch, the command used is "SW1(config)# vlan vlan-number." This command is specifically designed to allow the network administrator to create and configure a VLAN with the specified VLAN number. When this command is executed, the switch transitions into VLAN configuration mode, enabling the administrator to set parameters such as the name of the VLAN and any other attributes related to it.

In VLAN configuration mode, the syntax requires the user to specify the VLAN number directly after the 'vlan' keyword, which differentiates it from other configuration modes. This is integral for creating a new VLAN or modifying an existing one within the switch's configuration.

The other options provided do not lead to VLAN configuration mode correctly. For instance, using "interface vlan-number" typically refers to entering interface configuration mode for a specific VLAN interface, not directly configuring VLANs. The "set vlan" command is not valid in the Cisco IOS syntax for configuring VLANs, and "config vlan" does not correspond to any known command in IOS for VLAN configuration. Overall, the "vlan vlan-number" command is the correct and standardized method to manage VLANs on Cisco switches.