Which letter-cluster will come in place of the question mark (?) in the given series and complete the series? YWTZ, UTRY, ?, MNNW, IKLV

Find out the missing letter in the given series: A, Z, B, Y, C, X, ?, W, E

Which cluster will replace the question mark (?) in the following series? UBE 14, IPS 28, WDG 42, KRU 56, ?

Select the correct option to complete the repeating letter series: _ a _ d b _ a c d _ b a _ d b b _ c d