In the following letter series aa_ab__aaa_a, some letters are missing. Which one set of letters, when placed sequentially in the blanks, will correctly complete the series?

Introduction / Context:
This question is based on completing a pattern of letters where only two symbols, a and b, are used. The given sequence aa_ab__aaa_a includes four blanks that must be filled in a way that preserves a hidden structural rule. Problems of this type check a candidate's ability to see symmetry, repetition, and balanced arrangements without relying on numeric calculations. They are common in alphabet-test sections of aptitude exams.

Given Data / Assumptions:

• The partial series is written as: a a _ a b _ _ a a a _ a.• There are four blanks to be filled with a or b using one of the given sets.• The options supply sequences of four letters that must be inserted in order.• The completed pattern should look orderly and follow a consistent internal rule.

Concept / Approach:
We first rewrite the positions: 1 a, 2 a, 3 blank, 4 a, 5 b, 6 blank, 7 blank, 8 a, 9 a, 10 a, 11 blank, 12 a. Visual inspection suggests clustering of a letters at both the beginning and the end, with b letters concentrated in the middle. Therefore, we are looking for a completion that preserves this structure: a long run of a letters on each side, a compact group of b letters in the central region, and minimal irregular breaks.

Step-by-Step Solution:
Insert option d (a b a b) into the four blanks at positions 3, 6, 7 and 11.The completed sequence becomes: a a a a b b a a a a b a.Now observe the structure: there are four consecutive a letters at the start, then a pair of consecutive b letters, then another block of four consecutive a letters, followed by b a at the end.This forms a symmetric pattern with equal-length runs of a letters on both sides of a central double b cluster, something not achieved by the other options.

Verification / Alternative check:
If we try the other options, the completed strings look irregular. Option a produces aaaababaaaaa which has uneven alternations between a and b in the centre. Option b results in aaaabaaaaaba which creates a very long uninterrupted run of a letters in the middle and breaks symmetry. Option c yields aababaaaaaba, which begins with a short irregular alternation and does not match the balanced design seen in option d. Only option d creates a clearly structured arrangement: four a letters, two b letters, four a letters, then b a.

Why Other Options Are Wrong:
Options a, b and c all lead to patterns where the runs of a and b letters are either unbalanced, poorly grouped, or lack a recognisable structure. For example, some completions create only isolated single b letters or scatter b letters in a way that does not respect the natural central grouping hinted by the fixed letters. Since logical series questions generally follow a simple, symmetric rule, these irregular patterns are not suitable answers.

Common Pitfalls:
Students may try to memorise standard patterns instead of focusing on the specific arrangement in the question. Another pitfall is checking only how many a or b letters there are overall, instead of examining how they are grouped and ordered. A careful position-wise examination of the finished string is the safest way to determine the best completion.

Final Answer:
a b a b