The correct answer is $\boxed{\text{(B)}}$.
We know that K is the brother of N and X, Y is the mother of N, and Z is the father of K. This means that K and N have the same mother, and K and X have the same father. However, we do not know whether Y is the wife of Z. It is possible that Y is the mother of K and N but is not married to Z. For example, K and N could be half-brothers, or K could be adopted.
Here is a diagram that shows the possible relationships between K, N, X, Y, and Z:
[Diagram of a family tree with K, N, X, Y, and Z]
In this diagram, K and N are brothers, X is K’s son, Y is K’s mother, and Z is K’s father. However, this is just one possible relationship. There are many other possible relationships that would still be consistent with the information given in the question.
For example, K and N could be half-brothers, with different fathers. In this case, Y would be the mother of both K and N, but she would not be married to Z. Or, K could be adopted. In this case, Y would not be K’s biological mother, but she would still be his mother in the sense that she raised him.
Therefore, the only statement that is not definitely true is $\boxed{\text{(B)}}$, Y is the wife of Z.