Out of the four letter-clusters listed, three are alike in some manner and one is different. Select the odd one.

Let's check the numerical difference between the letters in each cluster (Forward sequence position values):NK: $N(14)-K(11)=3$ QA: $Q(17)-A(1)=16$ XU: $X(24)-U(21)=3$ GD: $G(7)-D(4)=3$ All option pairs have a difference of $3$ letters except for QA, which makes it the odd cluster out.