Given information:
- Family of 5 members: K, L, M, N, and O
- One married couple exists
- O is unmarried and is the brother of K
- N is the sister of O
- M is the only married female and the mother of N
- L and O are the only males in the group
Since L and O are the only males, the remaining members K, M, and N must be females.
O is unmarried and is the only other male besides L.
Since there is one married couple and M is the only married female, the married couple must be M and L.
M is the mother of N.
N is the sister of O, which means N and O are siblings with the same parents.
Therefore, M is also the mother of O.
O is the brother of K, which means O and K are siblings with the same parents.
Since M is the mother of O, M is also the mother of K.
Since M is the mother of K, N, and O, and L is married to M, L must be the father of all three children.
Therefore, the father of K is L.
The answer is Option 4: L