Choose the correct order of the given alternative : _ bcdbc _ dcabd _ bcdbc _…

This is a grouped letter-series item: the given twenty-four letters split into six blocks of four, and consecutive blocks are related by a fixed rule — each block's letters are the previous block's 2nd, 3rd, 1st and 4th letters, in that order — so every block is simply a rearrangement of the same four distinct letters, and the three-block pattern this creates then repeats. To solve for a blank, use the block that is already fully known to fix the missing letter, then check that value keeps every block a valid rearrangement of the same four letters.

1. Split the given twenty-four letters into six blocks of four: (_, b, c, d), (b, c, _, d), (c, a, b, d), (_, b, c, d), (b, c, _, d), (c, _, b, d).

2. The third block is given in full — c-a-b-d — with no blank, so it fixes the set of four distinct letters used throughout the series: a, b, c and d.

3. Every other block already shows three of those four letters and is missing exactly one: block 1 shows b, c, d and is missing a; block 2 shows b, c, d and is missing a; block 4 and block 5 repeat the same pattern as blocks 1 and 2; block 6 shows c, b, d and is missing a.

4. Since each block must be a rearrangement of all four letters a, b, c and d (as the fully given third block confirms), the one letter missing from every blank-containing block is exactly what each blank must supply.

As an independent check, the stated transformation rule (each block = 2nd, 3rd, 1st, 4th letters of the one before it) should also hold once the blanks are filled: with every blank read as 'a', the six blocks read abcd, bcad, cabd, abcd, bcad, cabd — each is exactly the 2nd-3rd-1st-4th rearrangement of the block before it, the three-block pattern abcd/bcad/cabd repeats exactly once, and block 6 comes out identical to the fully given block 3 — confirming the same result both ways.

So the same letter fills every blank, and that letter is 'a' — option aaaaa.