Step 1: EU Expansion.
Croatia became the 28\textsuperscript{th} member of the European Union on July 1, 2013, following a long process of negotiations and reforms.
Step 2: Analysis of options.
- (1) 2010: Croatia did not join the EU in 2010.
- (2) 2011: This is not the correct year for Croatia's accession.
- (3) 2013: Croatia officially became the 28\textsuperscript{th} member of the EU in this year.
- (4) 2014: Croatia joined the EU in 2013, not 2014.
Step 3: Conclusion.
The correct answer is (3) 2013, as that is the year Croatia became the 28\textsuperscript{th} member of the EU.