If you want to answer my questions, put up your hands....(one question)..(A boy answers the question, but not in a complete form....)

Question

1. Try to answer the question in a full sentence/form.

2. You should answer the question in full sentences/forms.

3. Answer the question in a complete sentence/form.

4. Will you answer the question in complete sentences/forms?

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Which expressions are suitable in the situation?

Answer 1

I would use #2 and/or #3, and I'd omit the word "form" or "forms."

I wouldn't use #1 or #4 because the others are very direct. Students often need direct instruction if they haven't performed as expected at first.