Question

Marque a forma interrogativa para a frase: "They study French once a week." *

They study French once a week?

They do study French once a week?

Does they study French once a week?

Do they study French once a week? ​

Answer 1

Do they study French once a week?

Do= I, you, we, they
Does= he, she it

Answer 2

O pronome they e acompanho de do segundo a o presente do verbo “to do”.
I do
You do
He, She, It does
We do
They do

Para formar a pergunta ocorre a mudança na posição do “do” para o início da frase, dessa forma a frase correta seria:
“Do they study French once a week?”

Espero ter ajudado : )