Which letter-cluster will replace the question mark (?) to complete the given series?

LMR, PUA, TCJ, XKS, ?

In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
_q_rpq_r_qq_

Which letter-cluster will replace the question mark (?) to complete the given series?

LE, OJ, UO, DT,?

Which letter-cluster will replace the question mark (?) to complete the given series?

QRM, PWD, OBU, NGL, ?

Which letter-cluster will replace the question mark (?) to complete the given series?

ABCD, HGLO, OLUZ, VQDK, ?

Which letter-cluster will replace the question mark (?) to complete the given series?

QML, PRC, OWT, NBK, ?

Select the letter that can replace the question0a mark (?) in the following series.
K, B, N, D, Q, F, T, H,?

In each of the following letters series, some of the letters are missing which are given in that order as one of the alternatives below it. Choose the correct alternative.
aa _ _ aba _ bca _ aab _ a _