"The boy writes his answer."
Translation:Cậu bé viết đáp án của mình.
"Câu trả lời" and "Đáp án" shared the same meanings in the daily life. You can replaced one with each other because these two words in Vietnamese are called "Từ đồng nghĩa". Example: If you say "Đưa tôi câu trả lời" or "Đưa tôi cái đáp án", Vietnamese people will understand you the same way. (If you don't understand anything because of my bad English, just let me know and I will find a way)
"cậu bé" is the way an adult would call a young boy, a fellow child would not use "cậu bé". "anh ấy" is used to refer to someone around the speaker's age or a few years older. so having "cậu bé" and "anh ấy" referring to the same person is kind of a contradiction, it has to imply two different people.
you could use "của cậu ấy" instead of "của mình" though.
This is the first we're hearing that anh ay is not a generic "he."!!!!!! Been working on my Vietnamese for going on four years now!!!!!!!!!!!!!!! This is also infuriating. I've known most my life that "anh" is age appropriate, but from the start Duo has said that "anh ay" is "he," and "cua anh ay" is his, I HAVE NEVER ONCE SEEN THEM USE "cua cau ay"!!!!! Your explanation makes perfect sense, I am not disbelieving you, I am just majorly disappointed that we've heard nothing about this before!