Explanation
The letter series can be analyzed by dividing it into groups of four letters. By examining the fixed positions of the letters b and c, we can identify an arithmetic pattern for their placement.
The letter b is positioned at indices 6, 10, 14, and 18. This indicates that b occurs at every position x where x = 4n - 2. Following this rule, the first gap at position 2 must be b.
The letter c is positioned at indices 3 and 11. This suggests a pattern where c occurs at positions x = 4n - 1. Following this rule, the second gap at position 7 should be c, the fourth gap at position 15 should be c, and the sixth gap at position 19 should be c.
Comparing these requirements with the given options:
Option a begins with c, which contradicts the requirement for b at position 2.
Option c begins with a, which also contradicts the requirement for b at position 2.
Option d begins with b, but its second letter is b, which would place b at position 7. As established, position 7 belongs to the sequence for c (4n-1).
Option b (bcaacc) correctly places b at position 2 and c at position 7.
By filling the series with the letters from option b, we get:
a (b) c a / a b (c) a / (a) b c a / a b (a) a / (c) b (c) a
In this sequence, the pattern abca repeats for the first three blocks. In the final two blocks, the total count of each letter is maintained (a:b:c in a 2:1:1 ratio), although the specific positions of one a and one c are swapped between the fourth and fifth blocks. Option b is the only choice that maintains the structural consistency of the first half of the series.