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 is interrogative, so the auxiliary 'had' functions as the question word, such that the structure is:
auxiliary/question word 'had' + subject 'he' + verb 'broken' + object 'his elbow' + prepositional phrase 'in the accident'.
The object 'his elbow' in the active voice now takes the place of the subject 'he', and vice versa. (subject + verb + object, where the subject and object swap places)
Hence the structure of the interrogative sentence in passive voice is: question word 'had' + object 'his elbow' + verb 'been broken' + prepositional phrase 'in the accident'. The verb 'broken' is changed to 'been broken', as the voice changes.
This structure is seen in option C, hence it's the correct option.
Option A: The structure here is incorrect, as 'been' is to be joined with 'broken' to form the passive voice. 'Been broken' must not be separated by the object 'his elbow'.
Option B: 'Be' is incorrect because it's in present tense whereas the sentence is in past tense.
Option D is incorrect because the answer lies in C.