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!