The question is asking for the country whose Prime Minister was assassinated in the year 2003. The options provided are Czech Republic, Romania, Serbia, and Slovenia.

Option 1 is the Czech Republic and though it went through significant political changes in 2003, its Prime Minister, Vladimír Špidla was not assassinated.

Option 2 is Romania. Adrian Năstase was the Prime Minister in 2003 and he was not assassinated.

Option 3, Serbia, is the correct answer. Zoran Đinđić was the Prime Minister of Serbia, and he was assassinated on March 12, 2003.

Option 4 is Slovenia, whose Prime Minister in 2003, Anton Rop, did not face any such fate.

So, Serbia is the correct response. The Prime Minister, Zoran Đinđić was the first democratically elected Prime Minister of Serbia who was assassinated in 2003.