Answer:
The correct answer is "Martin Luther King, Jr., fought for African Americans' rights".
Step-by-step explanation:
Active voice verbs are the ones which express actions made by the agent/subject. On the contrary, when the subject/agent has suffered an action, verbs are expressed on the passive voice form. The only sentence in the active voice form is the last one. All the other options are examples of passive voice forms. It is also possible to notice the passive voice construction made by "were brought" (in the first), "were helped" (in the second), and "was assassinated" (in the thrid), differently from the active voice construction made by "fought for" (in the last one).