Explanation:

- Let's analyze each option to find the pattern:

- EIO: If we look at their positions in the alphabet, E is the 5th letter, I is the 9th, and O is the 15th. These positions do not follow a particular arithmetic sequence.

- YCI: Y is 25th, C is 3rd, and I is 9th. Similarly, no particular arithmetic sequence is apparent.

- PTY: P is 16th, T is 20th, and Y is 25th. These numbers exhibit a clear arithmetic sequence, as they increase by 4 (16 + 4 = 20, 20 + 5 = 25).

- DHN: D is the 4th, H is the 8th, and N is the 14th. This does not follow a clear arithmetic sequence.

- PTY is the only one that forms an arithmetic sequence based on their positional values in the alphabet. Hence, PTY is the odd one out.