Based on the English alphabetical order, three of the following four letter-cluster pairs are alike in a certain way and thus form a group. Which letter-cluster pair DOES NOT belong to that group?
(Note: The odd one out is not based on the number of consonants/vowels or their position in the letter-cluster.)

Correct Answer: US−UV

To determine which letter-cluster pair does not belong to the group, let us analyze the pattern of the English alphabetical positions for each pair.

Let us write down the alphabetical position of each letter:

$\text{A}=1,\text{ B}=2,\text{ C}=3,\text{ D}=4,\text{ E}=5,\text{ F}=6,\text{ G}=7,\text{ H}=8,\text{ I}=9,\text{ J}=10,\text{ K}=11,\text{ L}=12,\text{ M}=13$

$\text{N}=14,\text{ O}=15,\text{ P}=16,\text{ Q}=17,\text{ R}=18,\text{ S}=19,\text{ T}=20,\text{ U}=21,\text{ V}=22$

Let us check the rule followed by each pair:

1. PN−QO:
First letter-cluster: P (16) and N (14) → P to N is -2.
Second letter-cluster: Q (17) and O (15) → Q to O is -2.
Relationship between clusters: P (16) to Q (17) is +1, N (14) to O (15) is +1.

2. QO−RP:
First letter-cluster: Q (17) and O (15) → Q to O is -2.
Second letter-cluster: R (18) and P (16) → R to P is -2.
Relationship between clusters: Q (17) to R (18) is +1, O (15) to P (16) is +1.

3. LJ−MK:
First letter-cluster: L (12) and J (10) → L to J is -2.
Second letter-cluster: M (13) and K (11) → M to K is -2.
Relationship between clusters: L (12) to M (13) is +1, J (10) to K (11) is +1.

4. US−UV:
First letter-cluster: U (21) and S (19) → U to S is -2.
Second letter-cluster: U (21) and V (22) → U to V is +1.
Relationship between clusters: U (21) to U (21) is 0, S (19) to V (22) is +3.

As we can see, options 1, 2, and 3 follow the exact same logic pattern of adding +1 to each letter to get the second pair. Option US−UV does not follow this pattern and is therefore the odd one out.