Explanation:

Let’s walk through each option and see what’s going on:

- Option 1: 321 : 213 : 123

- Take a look, these numbers are all rearrangements of one another—they’re anagrams with digits. Mix up 321, you can get 213 or 123.

- Option 2: 432 : 324 : 231

- Similar concept: Try to rearrange 432. You can get 324, but 231 isn’t a rearrangement of those same digits (the digits 4 doesn’t appear).

- Option 3: 798 : 897 : 798

- These are definitely rearrangements or repeats. 798 and 897 use the same digits, and 798 appears twice.

- Option 4: 564 : 645 : 456

- Again, easy to see—all the same digits, just shuffled.

Here’s the thing, Option 2 stands out because 231 doesn’t fit in with the other two (432 and 324); it throws in a 2 but drops the 4, breaking the pattern.

Answer:

Option 2 is the odd one out.