Which word makes the truest sentence? a man is always to his son?

The correct answer is "Related".

Explanation:  

In terms of grammar, we call a true sentence to that sentence that is factually correct. In that sense, the word "related" makes the sentence the truest one and it is more accurate to use that one. In terms of social, the word "related" makes the connection between the two people who are family.

the above statement/question is (true);

answer; integrative;