How to use tenses

Hello everyone,
Could you please give me the answer and the explanation for this.

They______the exercise when their teacher comes back.
A.are doing B.will be doing D.would do

Thank you.

To me, ‘D’ (would do) works best.
‘A’ and ‘B’ do not work because they are in present and future continuous forms.
‘C’ does not work as it is in present simple.
‘D’ can work either with ‘will do’ or with ‘would do’ since the activity is expected to start with the arrival of the teacher, both to take place in future.

A bit vague this test - I would plump for ‘will be doing’.

Hi Anhminh,
As suggested, there is more than one plausible correct answer to that question, though the options themselves are not exactly satidsfactory, so I would say the test is faulty.
The best option would be “will do” but that is not included.