Question

You must return that ' anna' to me with interest (rewrite using modal auxiliary showing advice)

Answer 1

You should return that 'anna' to me with interest.

Answer 2

You ought to return that 'anna' to me with interest. -This is the answer.

About modal verbs and their usage:

• Auxiliary verbs/ modal verbs are supporting verbs that come along with the main verb. 
• They support action verbs.
• Some of the auxiliary verbs are: shall, will, may, might, has, had, am, was etc.,
• These verbs always have an action verb following them.
• In the given sentence we have the modal verb 'must', which shows obligation. We have to replace it by a modal verb that shows an advice.
• Modal verbs that show advice are: ought to and should.
• Thus we have to replace 'must' with 'ought to'.
• Therefore the answer is: You ought to return that 'anna' to me with interest.