Based on the position in the English alphabetical order, three of the following letter clusters are alike in some manner and one is different. Select the odd letter-cluster.

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first and second letters are opposite letters.

Third and fourth letters are opposite letters.

By option (a) AZCX

A and Z are opposite letters.

C and X are opposite letters.

By option (c) PKRI

P and K are opposite letters.

R and I are opposite letters.

By option (c) MNOL

M and N are opposite letters.

O and L are opposite letters. 

Thus, correct option is (b).