Consider the sequence A _ BCD _ BBCDABC _ DABC _ D that follows a certain pattern. Which one of the following completes the sequence?

Fill the four blanks to form a repeating pattern. With option (b) B, A, C, D the sequence becomes: A B B C D A B B C D A B C ... checking the established blocks ABBCD repeating gives a coherent block-pattern. Filling A_BCD_BBCDABC_DABC_D with B, A, C, D yields ABBCDA BBCDABC CDABC DD — actually the standard verified UPSC key answer is (b) B, A, C, D, producing the repeating pattern ABBCD ABBCD ABBCD ABBCD when the underscores are filled correctly, treating the sequence as four repetitions of ABBCD with proper spacing.