The correct option is
D The bull was pulled violently.
The given sentence is in active voice as the subject 'someone' performs an action 'pulled' against the object 'bull'.
To change to passive voice, the object takes the place of the subject and vice versa and the word
'by' is introduced as now the subject receives the action by the object.
The verb form of the sentence is also changed from active form to
passive.
'Subject (Someone) + verb (pulled) + object (the bull) + adverb (violently)' changes to:
'New subject (The bull) + verb (was pulled) + adverb (violently)'
The
passive form of 'pulled' in the simple past tense is 'was pulled'.
This can be seen in option D, hence it's the right answer.
The pronoun 'someone' is indefinite, it does not refer to a particular person. Hence in passive voice it can be implied.
Option A: The verb form 'had been pulled' expresses the past perfect tense whereas the sentence is in simple past tense.
Option B: This sentence has changed in meaning. The verb phrase 'was to be pulled' means that it was meant or supposed to be pulled. The original sentence does not convey the same meaning.
Option C: Similarly, 'had been pulled' is in the past perfect tense.
Hence options A, B and C are incorrect.