Consider the sequence ABC_ _ABC_DABBCD_ABCD that follows a certain pattern. Which one of the following completes the sequence?

The sequence, when completed with option D (DDCA), becomes: A B C D D A B C C D A B B C D A A B C D

This 20-letter sequence can be analyzed by dividing it into four segments, each exhibiting a pattern of a repeated letter:

1. A B C D D: Here, the letter 'D' is repeated. (This fills ABC_ _ with DD)
2. A B C C D: Here, the letter 'C' is repeated. (This fills ABC_D with C)
3. D A B B C D: Here, the letter 'B' is repeated. (This is the fixed segment DABBCD)
4. A A B C D: Here, the letter 'A' is repeated. (This fills _ABCD with A)

The pattern is that each segment contains one letter repeated twice, and this repeated letter systematically follows the reverse alphabetical order: D, C, B, A.

Let's map this back to the original sequence with the blanks:

• The first part "ABC_ _" becomes "ABCDD" (repeated D). So, the first two blanks are DD.
• The next part "ABC_D" becomes "ABCCD" (repeated C). So, the third blank is C.
• The next fixed part is "DABBCD". This segment already has 'B' repeated, fitting the pattern of the repeated letter.
• The last part "_ABCD" becomes "AABCD" (repeated A). So, the fourth blank is A.

Thus, the blanks are filled by DDCA, and the entire sequence follows the pattern of a progressively shifting repeated letter (D, C, B, A).

The final answer is D.