In this series, third letter is repeated as first letter of next segment. The middle letter, A, remains static. The third letters are in alphabetical order, beginning with R.

There are three series to look for here. The first letters are alphabetical in reverse: Z, Y, X, W, V. The second letters are in alphabetical order, beginning with A. The number series is as follows: 5, 4, 6, 3, 7.

In this series, letters remain same: DEF.

The subscript numbers follow this series:

111, 112, 122, 222, 223, 233, 333, ...

Because letters are same, concentrate on number series, which is a simple 2, 3, 4, 5, 6 series, and follows each letter in order.

The first two letters, PQ, are static. The third letter is in alphabetical order, beginning with R. The number series is in descending order beginning with 5.