The correct option is B Will be sung
A sentence is said to be in the active voice when the subject performs the action.
A sentence is said to be in the passive voice when the subject receives the action.
Option A: 'Was sung' refers to a past action, something that has already happened. But, the action 'to be sung' is yet to happen, in the future "next week". Hence option A is incorrect.
Option B: 'Will sing' is an active verb phrase, it indicates that the subject is performing the action. But, the given sentence is passive, hence B is incorrect.
Options D and C: 'Shall' in the second or third person, refers to a threat or command. It isn't very suitable here.
In comparison, 'will' is a more suitable verb, as in the third person narrative (it) 'a prayer', 'will' indicates a future action.
Hence option C is correct and D is incorrect.