Everyone should be careful of their behavior.哪里错了?

Everyone should be careful of their behavior.哪里错了?
Everyone should be careful of their behavior.哪里错了?

Everyone should be careful of their behavior.哪里错了?
be careful about 当心,注意

没有错；
Everyone的谓语虽然要用单数，但后面的代词可用复数形式，所以their是对的；
behavior不可数，所以不加s也是对的。

Everyone should be careful of their behaviors
Everyone should be careful of his behavior

behavior变成behaviors
behavior可数，他们的举止，所以要用复数。