Explanation:

Let’s look at each option and try to find if there's a common pattern applied:

- Option 1: 81 : 19

8 + 1 = 9, 9 × 2 + 1 = 19

Possible logic, or something similar.

- Option 2: 21 : 23

2 + 1 = 3, 3 × 2 + 1 = 7 (but result is 23, so maybe direct addition?)

21 + 2 = 23

Here, just direct addition.

- Option 3: 1 : 3

Adds 2 to 1 = 3

- Option 4: 64 : 9

6 + 4 = 10, 10 × 2 + 1 = 21 (but result is 9)

64 + 2 = 66 (not matching), so doesn’t fit addition pattern.

- The common pattern in pairs 2 and 3 is "add 2" to the first number. Pair 1 does not fit that exactly but seems closer than option 4. In option 4, there is no logic connecting 64 and 9 similar to the other pairs.

- Correct answer:

Option 4: 64 : 9 is the odd pair.