Select the option that represents the letters that, when sequentially placed from left to right in the following blanks, will complete the letter series.
Z _ _ O _ _ E _ _ F _ E M _ F

The given series has 16 positions: Z _ _ O _ _ E _ _ F _ E M _ F.
Let's try substituting the options to see if a repeating block forms.

Let's test Option D: E, M, F, Z, M, O, Z, O
Placing them in the blanks:
Z E M O F M E Z M F O E M Z F O

Let's count the total letters: 16 letters. Let's break it into 4 groups of 4 letters:
Z E M O
F M E Z
M F O E
M Z F O
This doesn't seem perfectly regular. Let's try another grouping or check option A.

Let's test Option A: E, M, Z, F, M, O, Z, O
Z E M O | Z F E M | O F Z E | M O F

Let's test Option C: E, Z, F, M, O, Z, M, O
Z E Z O | F M E O | Z F M E | M O F

Let's analyze the pattern of a moving or shifting set of letters: Z, E, M, O, F?
Notice the sequence of groups:
If we fill it with Option D:
Z E M O
F M E Z
This doesn't match perfectly. Let's re-verify the letters from Option D:
Blanks: 1, 2, 3, 4, 5, 6, 7, 8
Option D sequence: 1=E, 2=M, 3=F, 4=Z, 5=M, 6=O, 7=Z, 8=O
Let's substitute them:
Z [E] [M] O [F] [M] E [Z] [M] F [O] E M [Z] F [O]
Let's read this:
Z E M O
F M E Z
M F O E
M Z F O -> wait, the last letter given in question is F, so it becomes M Z F F? No, the question says: Z _ _ O _ _ E _ _ F _ E M _ F
Let's count blanks:
Z (1) (2) O (3) (4) E (5) (6) F (7) E M (8) F
Total blanks = 8.
Let's check the letters with Option D:
1=E, 2=M, 3=F, 4=Z, 5=M, 6=O, 7=Z, 8=O
Resulting string: Z E M O F Z E M O F Z E M Z F O -> Wait, let's map carefully:
Positions:
1: Z
2: _ (E)
3: _ (M)
4: O
5: _ (F)
6: _ (Z)
7: E
8: _ (M)
9: _ (O)
10: F
11: _ (Z)
12: E
13: M
14: _ (O)
15: F
Wait, the template has: Z _ _ O _ _ E _ _ F _ E M _ F
Let's count characters:
Z (1) (2) O (3) (4) E (5) (6) F (7) E M (8) F -> This has 8 blanks.
Let's place Option D:
Z E M O F Z E M O F Z E M O F
Wow! This perfectly forms the repeating unit Z E M O F!
Let's verify:
Block 1: Z E M O F
Block 2: Z E M O F
Block 3: Z E M O F
Let's combine them: Z E M O F Z E M O F Z E M O F
Let's compare with the question text by replacing:
Z [E] [M] O [F] [Z] E [M] [O] F [Z] E M [O] F
Let's match the blanks:
Blank 1 = E
Blank 2 = M
Blank 3 = F
Blank 4 = Z
Blank 5 = M
Blank 6 = O
Blank 7 = Z
Blank 8 = O
The sequence of blanks is: E, M, F, Z, M, O, Z, O.
This matches Option D exactly.