Answer:
The best answer is "A hero is someone who sacrifices himself for a greater cause".
Step-by-step explanation:
The topic sentence is "A hero is someone who sacrifices himself for a greater cause" because it contains all keywords related to the other sentences, which means that the other options are only arguments and/or examples for this first main statement. These sentences could be easily organized in the following order: "A hero is someone who sacrifices himself for a greater cause", "For example, Mahatma Gandhi sacrificed himself to liberate India", [but] "Just because a person is oppressed does not make him a hero", "Self-sacrifice is a necessary ingredient of heroism".