Select the option that represents the letters that, when placed from left to right in the following blanks, will complete the letter-series.

S _ U C T S R _ _ T _ R U E _ S R _ _ T

To solve the letter-series, let us analyze the given pattern containing blanks:
S _ U C T S R _ _ T _ R U E _ S R _ _ T

First, let's count the total number of letters and blanks in the series. The given sequence is:
S (1) _ (2) U (3) C (4) T (5) S (6) R (7) _ (8) _ (9) T (10) _ (11) R (12) U (13) E (14) _ (15) S (16) R (17) _ (18) _ (19) T (20)
There are 20 positions in total. A series of 20 elements is typically divided into equal groups of 5 elements each.

Let us divide the series of 20 elements into 4 groups of 5 elements each:
Group 1: S _ U C T
Group 2: S R _ _ T
Group 3: _ R U E _
Group 4: S R _ _ T

Let us compare the groups to find the repeating pattern:
- Comparing Group 2 (S R _ _ T) and Group 4 (S R _ _ T), we see they are identical.
- Comparing Group 1 (S _ U C T) and Group 3 (_ R U E _), we see a strong similarity. If we align them:
Group 1: S _ U C T
Group 3: _ R U E _
If the first letter of Group 3 is 'S' and the last letter of Group 3 is 'T', we get:
Group 1: S _ U C T
Group 3: S R U E T
This suggests that Group 1 and Group 3 follow a pattern where the letters are changing slightly (specifically, the 4th letter changes from C to E, which represents an increase of 2: C + 2 = E). The second letter in Group 3 is 'R', so the second letter in Group 1 must also be 'R'.
This gives:
Group 1: S R U C T
Group 3: S R U E T
Here, the pattern is S R U _ T, where the 4th letter increases by 1 in each alternate group (C for Group 1, D for Group 2, E for Group 3, F for Group 4).
Let's check this hypothesis by applying the changing 4th letter across all four groups:
Group 1: S R U C T
Group 2: S R U D T
Group 3: S R U E T
Group 4: S R U F T

Let us verify if this fits the original series with the blanks:
- Blank 1 (2nd position, in Group 1): R
- Blank 2 (8th position, in Group 2): U
- Blank 3 (9th position, in Group 2): D
- Blank 4 (11th position, in Group 3): S
- Blank 5 (15th position, in Group 3): T
- Blank 6 (18th position, in Group 4): U
- Blank 7 (19th position, in Group 4): F

Putting these letters together from left to right gives:
R U D S T U F

Thus, the correct option that completes the series is R U D S T U F.