In the given sentence, option B is correct as it signifies necessity, compulsion and certainty. It fits in the context of the sentence.
Option D \'has to\' here is incorrect as it is in singular form and \'they\' in the given sentence represents plural form.
Option A \'should\' is incorrect here as \'Should\' shows suggestion, and the tone in the sentence signifies compulsion of keeping the passports ready.
Ought expresses ideas such as duty, necessity and moral obligation. It is not as forceful as must, but stronger than should. Therefore Option C is also incorrect.
The correct option is B.
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