Which letter-cluster will replace the question mark (?) to complete the given series?
TUWA, ?, RISI, QEQO, PAOU

Correct option is B

Given - TUWA, ?, RISI, QEQO, PAOU
First letter – T, ?, R, Q, P
Logic: letters are in decreasing order.
T – 1 = S
S – 1 = R
R – 1= Q
Q – 1 = P
Second letter – U, ?, I, E, A logic: vowels are in reverse order
? = O
Third letter – W, ?, S, Q, O
Logic: letters are decreasing by 2
W – 2 = U
U – 2 = S
S – 2 = Q
Q – 2 = O
Fourth letter – A, ?, I, O, U
Logic: - all vowels are in actual otder.
? = E
Finally letter – cluster ‘SOUE’ will replace the questions mark.
Option (b) SOUE is the correct answer.