Randomising responses within a bot is super easy. In the top, right corner of the platform, you will see the button "Multi response messages".
This feature will let you assign multiple messages to a single option. You can pick whether you want the bot to randomise these messages, or if you want them to go in sequential order.

Randomising responses is especially useful when using the default reply. It prevents bot content becoming stale, especially if your bot heavily relies on keywords.