Question

Modal auxiliary verb. You___ tell me the truth for your own good
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Answer 1

Answer:

Should is right answer

Answer 2

You should/must tell me the truth for your own good.

There are nine modal auxiliary verbs: shall, should, can, could, will, would, may, must, might. The goal of the modal auxiliaries is to represent possibility (hypothesis, futurity, uncertainty) and necessity (by inference, such-and-such must necessarily be the case); in other words, things that aren't true right now.

As the speaker intends obligation both the modal auxillaries should and must can be used.