Select the set of letters that, when sequentially placed in the blanks of the given letter series, will complete the letter series.
u _ q _ c _ u r _ z _ n _ r q _ c n

Let's find the length of the string: there are 18 positions.
Let's try to divide this series into periodic repeating sub-patterns. Since 18 can be factored into groups of 6:
u _ q _ c _ | u r _ z _ n | _ r q _ c n

Let's compare the three groups to build a unified string:

Group 1: u _ q _ c _

Group 2: u r _ z _ n

Group 3: _ r q _ c n

By matching corresponding positions across the groups:

Position 1: u (from groups 1 & 2) → Group 3 starts with u.

Position 2: r (from groups 2 & 3) → Group 1 second letter is r.

Position 3: q (from groups 1 & 3) → Group 2 third letter is q.

Position 4: z (from group 2) → Groups 1 and 3 fourth letter is z.

Position 5: c (from groups 1 & 3) → Group 2 fifth letter is c.

Position 6: n (from groups 2 & 3) → Group 1 sixth letter is n.

Thus, the repeating pattern is u r q z c n.

Let's identify the missing blanks based on this pattern:

Group 1 (u r q z c n): blanks are at position 2 (r), position 4 (z), position 6 (n).

Group 2 (u r q z c n): blanks are at position 3 (q), position 5 (c).

Group 3 (u r q z c n): blanks are at position 1 (u), position 4 (z).

Putting the inserted letters together in sequence: r, z, n, q, c, u, z.
This aligns with Option C.