To solve this question, we need to identify the pattern in the given letter series and find the set of letters that can complete it.

Let's analyze the given letter series: _bcdbc_dcabd_bcdbc_dc_bd

Pattern Analysis:
- The first letter in the series is always a blank (_) followed by a letter 'b'.
- The remaining letters can be divided into groups of five: bcdbc, dcabd, bcdbc, dc, bd.
- Within each group of five letters, the first three letters are repeated twice, followed by two different letters.
- The last group of two letters is the same in each set.

Based on this pattern analysis, we can conclude that the missing letters in the series should also follow the same pattern.

The missing letters are represented by the blank (_) in the series.

Let's fill in the blanks with the options to see which one completes the series:

1) _bbbb_
2) _ddddd_
3) _ccccc_
4) _aaaaa_

Option Analysis:
1) _bbbb_: This option does not follow the pattern of the series. The first three letters in each group are not the same.
2) _ddddd_: This option follows the pattern of the series. The first three letters in each group are the same (d), followed by two different letters. The last two letters are the same.
3) _ccccc_: This option does not follow the pattern of the series. The first three letters in each group are not the same.
4) _aaaaa_: This option follows the pattern of the series. The first three letters in each group are the same (a), followed by two different letters. The last two letters are the same.

Conclusion:
Based on the pattern analysis and filling in the blanks with the options, we can see that option 'D' (aaaaa) completes the given letter series.