The correct answer is 'By whom was the tiger killed?'. These are the following steps to convert active voice into passive voice:-In the given sentence, as we already know who is used as the subject. Who will become the passive object. And the objective case of who is whom. So, while changing the voice, we'll use by whom as the passive object.Use a suitable be-verb after the passive subject. Here the passive subject is 'the tiger' and a suitable be verb for this subject is 'is' or 'was'. But as the tense is past tense, the past be verb 'was' will be used here.Use 3rd form of the main verb after the subject.[Here, killed is the v3 form of kill.]As we are making an interrogative sentence, we have to use the be-verb before the passive subject and the question mark(?) at the end of the sentence.Hence, the correct answer is option 3.