Thieu chu "có"
"Có" is often omitted, especially when speaking.
Why having có is wrong here? Isn't it yes/no question?
Hôn nhân của cô ấy có là hạnh phúc không?
It's "là" that made your answer wrong.
It worked for me.
Why is vui = hạnh phúc counted wrong here?
Think of "vui" as an equivalent of "glad." It'd sound strange to ask if someone's marriage is glad."