Fill in the blanks with the missing letters to complete the series:
$$\text{P\_OEN\_XPH\_E\_I\_PHO\_NIX\_HOEN\_X}$$

Let's count the total number of letters and spaces or look for a repeating meaningful word pattern.
We can notice the letters form variants of the word PHOENIX.

Let's divide the series into blocks based on the word length of PHOENIX ($7$ letters):

$$\text{P H O E N I X}|\text{P H O E N I X}|\text{P H O E N I X}|\text{P H O E N I X}$$

Let's verify the inserted letters sequentially:
1st blank: H
2nd blank: I
3rd blank: O
4th blank: N
5th blank: X
6th blank: E
7th blank: P
8th blank: I

The sequence of letters filled in the blanks is: H, I, O, N, X, E, P, I.
This matches Option A perfectly.