Select the letter-cluster that can replace the question mark (?) in the following series: QZR, NYO, KWL, ?, EPF

Which of the given letter-number clusters will replace the question mark (?) in the following series to make it logically complete? USO 48 NLH 39 GEA 30 ZXT 21 ?

Select the term from among the given options that can replace the question mark (?) in the following series. ZDF1, YGH3, XJJ9, WML27, ?

Which of the following letter-clusters will replace the question mark (?) in the given series to make it logically complete? ODH, QFJ, SHL, UJN, ?

Which of the following letter-clusters will replace the question mark (?) in the given series to make it logically complete? XLD, AOG, DRJ, GUM, ?

Which of the given letter-number clusters will replace the question mark (?) in the following series to make it logically complete? YCG 18 HLP 23 QUY 28 ZDH 33 ?

Which of the following letter-number clusters will replace the question mark (?) in the given series to make it logically complete? OSW 24, GKO 35, YCG 46, QUY 57, IMQ 68, ?

What should come in place of the question mark (?) in the given series based on the English alphabetical order? COB, GSF, KWJ, OAN, ?