Correct Answer: ET

Solution : Let's check the options –
First option: MN; Both letters are opposite to each other.
Second option: NM; Both letters are opposite to each other.
Third option: OL; Both letters are opposite to each other.
Fourth option: ET; Both letters are not opposite to each other as the opposite letter of E is V and the opposite letter of T is G.

So, the fourth option is different from the other options. Hence, the fourth option is correct.