Select the most appropriate Modal from the options given below to complete the blank in the given sentence:

Parents always advise their children that they ____ mix with those who have bad habits.

Must not

The correct answer is Option (4) → Must not

Must not is used to express prohibition or negative necessity. In this case, the parents are advising their children to not mix with those who have bad habits. This is a negative statement, so must not is the correct modal.

The other modals do not fit the context as well:

• Can is used to express possibility or ability. It would not be appropriate to use this modal in this case, as the parents are not saying that their children are able to mix with those who have bad habits.
• Must is used to express strong necessity or obligation. It would not be appropriate to use this modal in this case, as the parents are not saying that it is absolutely necessary for their children to not mix with those who have bad habits.
• Cannot is used to express impossibility or inability. It would not be appropriate to use this modal in this case, as the parents are not saying that their children are unable to mix with those who have bad habits.