The correct answer is Option 1: Bulgaria and Romania.

In January 2007, both Bulgaria and Romania officially joined the European Union (EU) as member states. This marked an important enlargement of the EU, expanding its membership from 25 to 27 countries.

Bulgaria, located in Southeastern Europe, had been in negotiations to join the EU since 2000. The country made significant efforts to align its legislation and policies with EU standards and requirements, particularly in areas such as judiciary, anti-corruption measures, and market reforms. After successfully meeting the necessary criteria, Bulgaria became an EU member on January 1, 2007.

Similarly, Romania, located in Eastern Europe, had been engaged in accession talks with the EU since 1995. The country implemented various reforms in areas such as the judiciary, administration, and agriculture to meet the EU`s political and economic requirements. After fulfilling the necessary conditions, Romania also joined the EU on the same date as Bulgaria, January 1, 2007.

This simultaneous accession of Bulgaria and Romania was the result of their shared commitment to EU integration, their progress in implementing necessary reforms, and the positive assessments of their readiness by the European Commission and other EU member states.

It`s important to note that the other options provided in the question are incorrect:

- Option 2: Bulgaria and Belgium: Belgium has been a member of the EU since its inception in 1957 and did not join in 2007 alongside Bulgaria.

- Option 3: Romania and Slovenia: While Romania did join the EU in 2007, Slovenia became an EU member earlier, in May 2004.

- Option 4: Hungary and Croatia: Hungary joined the EU in May 2004, along with several other Central and Eastern European countries. Croatia, on the other hand, became an EU member state later, on July 1, 2013.

Therefore, the correct answer is Option 1: Bulgaria and Romania, as they were the countries that joined the European Union in January 2007.