The correct option is
B to suffer
In order to fill in the blank with the most suitable verb, we need to understand the usage of 'make'. 'Make' means to force someone to do something. If 'make' is used in a passive (voice) sentence, we use the word 'to' along with the verb. In an active sentence, we do not use 'to' with the verb. The verbs without the marker 'to', are called bare infinitives.As the given sentence is in the passive voice, we will not use the bare infinitive here; we will use the verb with the marker 'to'.
NOTE: Here, 'made' is used as the main verb, which describes the sentence in past tense. Though the sentence is in past tense, the verb with the marker 'to' will always be in its original form.
Option B - As 'suffer' is a bare infinitive, we can't use it here. Hence, option A is incorrect.
Option C - 'Suffering' is the participle form of the verb 'suffer'. As explained above, we need a verb in its original form with the marker 'to'. Hence, option C is incorrect.
Option D - 'Suffers' is the third person form of the verb 'suffer'. As explained above, we need a verb in its original form with the marker 'to'. Hence, option D is incorrect.
Option A - 'To suffer' can be used here as the sentence is in the passive voice. The verb 'suffer' is in its original form and is accompanied by the marker 'to'. Hence, option A is the correct answer.
'They will be made to suffer by God.'