In the following triads, each group of letters is related to the subsequent one following a certain logic. Select from the given options the one which follows the same logic. WEST - EWST - WSTE TOPS - OTPS - TPSO

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