What significant change occurred in relation to the UK's membership in the European Union?

The significant change related to the UK's membership in the European Union is that the UK officially left the EU. This event is commonly referred to as "Brexit," which was formally executed on January 31, 2020, after a referendum held in June 2016. The decision to leave the EU marked a profound shift in the UK's relationship with Europe, affecting trade, travel, and numerous regulatory frameworks that had previously been governed by EU law. It initiated a period of negotiations to establish new agreements between the UK and EU on various matters, including trade, market access, and citizens' rights.

In contrast to the other options, introducing the Euro as currency would have implied deeper integration with the EU, which was not part of the decision reflected in Brexit. Expanding the UK's borders does not directly correlate with EU membership or departure, and closing all borders is an extreme measure that was not implemented as part of Brexit. The focus on maintaining a balance between border control and trade relationships has been a crucial part of the ongoing discussions post-Brexit. Therefore, the action of officially leaving the EU stands out as the primary and significant change in this context.