Arun applied the brakes of the car immediately. 

Change the sentence into passive voice.

When the verb's subject (in the sentence) receives the action named in the verb, the verb is said to be in the passive voice. According to the rule on changing active voice into passive, if a sentence is in the simple past tense to convert into a passive voice, the given sentence's helping verb has to change (helping verb "was/were"). Therefore, option B is correct.