The auxiliary, 'must' is used to express a rule or an obligation expressed by the speaker. The sentence, 'All personal belongings must be left in the foyer' means that this is a rule which needs to be followed. Hence, option C is the most appropriate choice.
The auxiliary, 'have' is used to form the perfect tenses. The sentence, 'All personal belongings have been left in the foyer' is a statement of fact about something which has already been done. It does not express a rule, which needs to be followed. Hence, option A is incorrect.
The auxiliary, 'can' is used to express ability/ capability. It also expresses permission to do something. The sentence, 'All personal belongings can be left in the foyer' means that 'Leaving personal belongings in the foyer is allowed'. However, it does not make it a compulsory rule to be followed. Hence, option B is incorrect.
The auxiliary, 'will' is used to express a volition or a choice intended to be exercised. It also expresses an action which is likely to happen in the future. The sentence, 'All personal belongings will be left in the foyer' means 'It is intended to keep all personal belongings in the foyer'. It does not, however, express a rule or a condition. Hence, option D is incorrect.