Three of the following four letter-clusters are alike in a certain way and thus form a group. Which letter-cluster does not belong to that group?

Logic: ​+2 then +3 in alphabetically.
Explanation:

A: P(16), R(18), U(21)
+2, +3 

B: R(18), T(20), W(23)
+2, +3 

C: M(13), O(15), R(18)
+2, +3 

D: F(6), H(8), L(12)
+2, +4 ✘

Three clusters follow pattern +2, +3. Only D does not.

Final Answer:
FHL

Final Correct Option:
(d)