Which of the following letter-clusters will replace the question mark (?) in the given series?
RI, PK, LO, FU, XC, ?

Let's analyze the pairs of letters in each cluster. Note that each pair consists of opposite letters (R↔I, P↔K, L↔O, F↔U, X↔C).
Now let's find the pattern tracking the first letter of each cluster:

R(18)→P(16)⟹−2

P(16)→L(12)⟹−4

L(12)→F(6)⟹−6

F(6)→X(24)⟹−8 (since 6−8=−2≡24)

Following this pattern of decreasing even numbers (−2,−4,−6,−8,−10), the next subtraction should be −10:

X(24)−10=14⟹N
The opposite letter of N is M.
Thus, the next letter-cluster in the series is NM.