Translation:I have not come to defend my husband, but my son.
The definite article precedes a possessive adjective but it can be left out when speaking of close relatives. It is often omitted in Brazil. The possessive pronouns do not require an article except when emphasizing ownership. So I guess the question could have used 'meu marido' and 'o meu filho' and the answer would still be the same as the one Duo gives.
"I have come to defend not my husband, but my son" would be the best way to say this in English, but it was counted wrong. The negation should apply only to the direct object, not to the verb. Of course English speakers don't necessarily apply the rules of logic, so plenty of people would say "I have not come..." or "I haven't come..." or "I didn't come..." in this context. But the logically correct version should not be counted wrong!
I think the point is that in informal speech people say "I have not come", in Brazil as well as in the US. The way the Portuguese is written in this case reflects that. It's nonsense, yes, but it's a more accurate translation of the "incorrect" idiom in Portuguese to the same idiom in English.
For an example, the best translation of Zolá's L'Assomoire (imo) translates the main characters' dialog as 19th-Century cockney dialect, to reflect the English equivalent to how a poor, uneducated French person who is from a rural area in the 19th Century would sound to a French reader.