Which letter-cluster will replace the question mark (?) in the given series to complete it? ALTY, DNVA, ?, JRZE, MTBG

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