those themes ( Twenty Fourteen to Twenty Twenty) are WordPress own default theme and WordPress always use the last version theme as default install theme. But if you are using Avada or any other premium theme then those Twenty Fourteen to Twenty Twenty themes are optional theme. you can remove/delete them from cpanel/file manager.
If you have any technical query about Avada theme then please contact the author of your purchased theme, the theme author will assist you.
How to contact an author to get support or for any technical query:
a) yes, right.
b) Onmouse over the Avada theme screenshots there you will get popup where a link ‘Theme Details’ which will show you the current version of theme you are using. If you like to see the last update info of the Avada theme then you can see it from the theme page at themeforest from your Download page.
c) yes but you can keep at least one Twenty Twenty.