Four letter-clusters have been given, out of which three are alike in some manner and one is different. Select the letter-cluster that is different.

Let's calculate the sum of the alphabetical positions of the letters in each cluster:Option A: GBI $\to$ G(7) + B(2) + I(9) = 18Option B: AJG $\to$ A(1) + J(10) + G(7) = 18Option C: GBK $\to$ G(7) + B(2) + K(11) = 20Option D: CHG $\to$ C(3) + H(8) + G(7) = 18Notice that the sum of the letter positions in options A, B, and D is exactly 18, whereas in Option C it is 20. Therefore, GBK is the different cluster.