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

L _ _ RALMT _ _ _ _ TRALM_ R _

The correct option is MTRALMTA.

To solve the letter series, let us first look at the given incomplete sequence with blanks:
L _ _ RALMT _ _ _ _ TRALM_ R _

Let us count the total number of letters and blanks in the sequence. The sequence contains 24 positions in total:
1. L
2. _
3. _
4. R
5. A
6. L
7. M
8. T
9. _
10. _
11. _
12. _
13. T
14. R
15. A
16. L
17. M
18. _
19. R
20. _

Wait, let's recount the characters in the given string:
"L _ _ RALMT _ _ _ _ TRALM_ R _"
Let's list them:
1: L
2: _
3: _
4: R
5: A
6: L
7: M
8: T
9: _ (first of the 4 underscores)
10: _
11: _
12: _
13: T
14: R
15: A
16: L
17: M
18: _ (underscore)
19: R
20: _ (underscore)
Let us check the total number of blanks. There are:
- 2 blanks after L (positions 2, 3)
- 4 blanks after T (positions 9, 10, 11, 12)
- 1 blank between M and R (position 18)
- 1 blank at the end after R (position 20)
Total number of blanks = 2 + 4 + 1 + 1 = 8 blanks.

The correct option has 8 letters: M T R A L M T A. Let's place these letters sequentially in the blanks:
- 1st blank (position 2): M
- 2nd blank (position 3): T
- 3rd blank (position 9): R
- 4th blank (position 10): A
- 5th blank (position 11): L
- 6th blank (position 12): M
- 7th blank (position 18): T
- 8th blank (position 20): A

Now, let us write down the completed sequence:
L M T R A L M T R A L M T R A L M T R A

Let's analyze this completed sequence:
L M T R A L M T R A L M T R A L M T R A

We can clearly see a repeating pattern of 5 letters:
L M T R A | L M T R A | L M T R A | L M T R A

Since the pattern "LMTRA" repeats continuously, the sequence is formed by repeating the block "LMTRA" four times. Thus, the letters that fill the blanks sequentially from left to right are indeed M, T, R, A, L, M, T, A.