Choose the correct order of the given alternative to complete the series: aab…

Concept

Many alphabet-series questions are built from equal-length blocks that each satisfy a fixed rule — commonly, every block contains all three letters in a small set, with exactly one of them appearing twice (doubled), and the identity of the doubled letter shifts in a fixed cyclic order from block to block.

Application

1. Split the 20-letter string into five blocks of four letters each: positions 1-4, 5-8, 9-12, 13-16, and 17-20.

2. The block with no blank (positions 9-12) reads abcc — one a, one b, and c doubled — so c is the doubled letter for that block.

3. Insert the correct answer's letters (c, b, a, b) into the four blanks in order.

4. Block 1 becomes aabc (a doubled), block 2 becomes abbc (b doubled), block 4 becomes aabc (a doubled again), and block 5 becomes abbc (b doubled).

5. Across the five blocks the doubled letter runs a, b, c, a, b — a steady repeating cycle, with every block containing all three letters and exactly one of them doubled.

Cross-check

The one block that was already complete (positions 9-12, reading abcc) sits between a b-doubled block and the cycle restarting at a-doubled — exactly where a c-doubled block belongs in the a-to-b-to-c cycle, confirming the rule independently of the filled blanks.

So the blanks must be filled with c, b, a, b, in that order.