A sentence is in active voice when the subject is the doer of the action.
A sentence is in passive voice when the new subject (object in active voice) is the receiver of that action.
The given sentence: "...so that Christmas ___ by these unfortunate people too." requires a verb in passive voice form.
Option B: 'May be celebrated' indicates a possibility, 'may' is a modal verb that refers to possibility. This is the most suitable option as it refers to a future event and the structure is the correct one for modal verbs for present/future tense in the passive voice: modal (may) + be + past participle (celebrated).
Hence B is correct.
Option A: 'Might be celebrated' also follows the same structure as 'may be celebrated', but the difference is that 'might' is used as the past tense of 'may' in Indirect speech. However, the given sentence does not indicate that it is in reported speech. Hence A is incorrect.
Option C: 'Must celebrate' is in the active voice. 'Must' indicates an obligation or compulsion. But, the sentence does not need an active verb. Hence C is incorrect.
Option D: 'Be celebrate' is grammatically incorrect. Both verbs are in their base forms 'to be', 'to celebrate' and are not conjugated according to its use in a sentence. Hence D is incorrect.