You have site versions for France and Italy and you set up two hreflangs for them. For the rest of your end-users you plan to use the English version of the site. Which directive will you use?

The hreflang="x-default" attribute is used to specify the default version of a page when no other language or regional URL matches the user's browser settings. In this scenario, you have specific hreflangs for France and Italy. For all other users, you want them to see the English version. x-default correctly designates this English version as the fallback for any user whose language or region isn't explicitly covered by your other hreflang tags. hreflang="en-au" is incorrect because it specifies English for Australia, not a general English fallback for all other users. hreflang="uk" is also incorrect as it's not a valid hreflang code; the correct code for English in the United Kingdom would be en-gb.