Son
Brother
Boy
Sister
Answer is Wrong!
Answer is Right!
The answer is son.
The relationship between mother and daughter is one of parent and child. The relationship between father and son is also one of parent and child.
The other options are incorrect because they do not represent a parent-child relationship.
A son is a male child. A brother is a male sibling. A boy is a young male. A sister is a female sibling.