Correct Option (d)

The given sequence is ABC_ _ABC_DABBCD_ABCD. The objective is to identify the missing characters that complete the sequence according to an underlying pattern.

Upon analysis, the complete sequence follows a systematic pattern comprising four distinct blocks:

Block 1: ABCDD
Block 2: ABCCD
Block 3: ABBCD
Block 4: AABCD

When these blocks are concatenated, they form the complete sequence: ABCDDABCCDABBCDAABCD.

Inserting the characters from option (d) DDCA into the blanks of the original sequence yields:

ABC DD ABC CD ABBC D A ABCD

Comparing this filled sequence with the established pattern:

The first two blanks are filled by DD, completing the first block as ABCDD.
The third blank is filled by C, completing the second block as ABCCD.
The fourth blank is filled by A, completing the third block as ABBCDA.

This confirms that the sequence DDCA correctly fills the blanks, adhering to the identified pattern.

The pattern demonstrates a progressive transformation across the blocks, where a character shifts one position backward in the alphabet (D→C, C→B, B→A) at progressively leftward positions within the block structure:

From ABCDD to ABCCD: The character at the 4th position changes from D to C.
From ABCCD to ABBCD: The character at the 3rd position changes from C to B.
From ABBCD to AABCD: The character at the 2nd position changes from B to A.

Incorrect Options:

Options (a) DACB, (b) CDAB, and (c) DCCA do not conform to the identified systematic pattern of block formation and transformation. Inserting these character sequences into the blanks would result in an inconsistent or disrupted pattern, thereby failing to complete the sequence correctly according to the established logical progression.