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: "But, Mr. Scrooge suggested that the poor ___ to prisons or poor houses." requires a verb in passive voice form.
Option C: 'Should be celebrated' refers to a future action, 'should' is a modal verb that refers to a suggestion. 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 (should) + be + past participle (sent).
Hence C is correct.
Option B: 'Shall be sent' also follows the same structure as 'should be sent', but the difference is that 'shall' is used in direct speech and 'should' is used as the past tense of 'shall' in Indirect speech. (Also, 'shall' as a modal does not indicate suggestion, as the sentence mentions: Mr. Scrooge suggested... )
The given sentence indicates that it is in reported speech: 'Mr. Scrooge suggested that...' Hence B is incorrect.
Option A: 'Being sent' sounds incomplete on its own. 'Being' is a present participle. It does not function as a verb in this sentence. An auxiliary verb is required to complete the meaning of the phrase 'being sent'. For example: 'is being sent', 'was/were being sent'. Hence A is incorrect.
Option D is incorrect because the answer lies in C.