ANSWER : abxdm

Explanation : . The group of five letters, ab x dm, has been repeated.

ANSWER : uvxyz

Explanation : . Xyzuv/yzuvx/zuvxy/uvxyz. In each block 1st letter of each previous block is taken to the end of the successive block.

ANSWER : abbbb

Explanation : . Abcde/abcdd/abccc/abbbb. In 2nd block, 5th letter has been replaced by 4th letter, in 3rd block, 4th letter has been replaced by 3rd letter and so on.

ANSWER : zuxyz

Explanation : . The pattern of the series of repetitive block is xyzu/xyzu/���

ANSWER : xyyyy

Explanation : . The pattern of the series is xy/xxyy/xxxyyy����.