Select the combination of letters that when sequentially placed in the blanks…

A letter-series completion item is built around one repeating block of letters: the full series is the same block written out several times in a row, with some letters shown and others left blank. The option whose letters, dropped into the blanks in the given order, make every occurrence of the block read identically is the answer — an option that matches in some occurrences but breaks the match in others is a distractor.

Here the 21-letter series splits into three 7-letter blocks with seven blanks in total: a _ b c d _ _ | a a b c _ d e | a _ _ c d _ e. Checking each candidate combination against the blanks (each case is independent of the others):

• dabdade → the three blocks come out a d b c d a b / a a b c d d e / a a d c d e e — the middle block matches a valid seven-letter reading, but the first and third blocks differ from it, so this candidate is rejected.

• adabded → the three blocks come out a a b c d d a / a a b c b d e / a d e c d d e — none of the three blocks match each other, so this candidate is rejected.

• adedabd → the three blocks come out a a b c d d e / a a b c d d e / a a b c d d e — all three blocks are identical, so this candidate is retained.

• adebdda → the three blocks come out a a b c d d e / a a b c b d e / a d d c d a e — only the first block matches that reading; the other two differ, so this candidate is rejected.

Reading the retained candidate's three completed blocks letter by letter — a-a-b-c-d-d-e in each case — confirms the same seven-letter unit repeats three times with no discrepancy, which is the defining test for this series type.

Hence, adedabd (the combination a-d-e-d-a-b-d) is the correct answer.