Let's eliminate our options and then check.
Since 1, 3, and 5 are odd numbers, there's a 50% chance for and then it gets reduced to a 25% chance due to the second number have to be odd.
This means that 9 of the pairs will have an even number, odd number duo.
The pairs are:
2,1
2,3
2,5
4,1
4,3
4,5
6,1
6,3
6,5
So, B. 1/4