What should come next in the following letter sequence?

A A B A B C A B C D A B C D E A B C D

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
NM, PK, SI, ?

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
ZA, TG, NM, ?

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
DAF, FED, HIB, ?

A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
DP, EY, FJ, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.
DE, GI, KN, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.
FU, LO, RI, ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

ENG, GQI, ITK, KWM, ?

Which of the following terms follows the trend of the given list?

XYZXYZxYZ, XYZXYzXYZ, XYZXyZXYZ, XYZxYZXYZ, XYzXYZXYZ, _______________.