Explanation
The series is completed by placing the letters from option c) into the blanks to form a repeating logical structure. By substituting the letters u, b, d, j, m, b, u, b, and a into the respective blanks, the full sequence becomes:
j u m b b d a j m u b b u a d j m b b a d
This sequence can be divided into three equal segments of seven letters each:
j u m b b d a
j m u b b u a
d j m b b a d
In this structure, a clear pattern emerges: the fourth and fifth positions of every seven-letter block are consistently occupied by the letters b and b. The remaining positions (1st, 2nd, 3rd and 6th, 7th) are filled using a rotating set of the letters j, u, m, d, and a.
Alternatively, the series can be viewed as a sequence of seven triplets:
(j u m), (b b d), (a j m), (u b b), (u a d), (j m b), (b a d)
Triplets 1, 3, and 6 contain the core letters j and m along with a third varying letter (u, a, and b respectively). Triplets 2, 4, and 7 contain at least one b and follow a shift in their third letter (d, u, and a respectively). Triplet 5 acts as a bridge using the remaining letters u, a, and d. Substituting the letters from option c) is the only way to satisfy these internal consistencies.