If you meant to say either
or B', which refers to the complement of set B, then we're talking about the set of stuff not found in set B. Every value mentioned is in the universal set (whether its in set B or not).
B = {13, 15, 17, 19, 23, ...}
Set B talks about odd numbers from 13 onward. The opposite of this would be odd natural numbers up to but excluding 13. In other words, we'll go from 1 to 11
B' = {1, 3, 5, 7, 9, 11}
Applying the set union to B and B' will get us the universal set.
Answer: {1, 3, 5, 7, 9, 11}