Puzzling over why contents ist not acceptable in place of heureux. There are no other errors in the sentence which aligns exactly with the expected response otherwise.
Il aime rendre tous ses frères contents is not something one would say in french. Rendre heureux is a kind of fixed expression and the same sentence with contents won't work.
Heureux and content are not perfect synonyms in french.
Jojo, is it because rendre heureux is a kind of fixed expression that we can't use the verb faire as a substitute for rendre?
Content is closer to "satisfied" than to "happy".
Oh, i used "tout", not "tous". With "tous", it's accepted.
Why is this not "rendre heureux tout ses frères"? In an earlier sentence, that structure was used.