The correct answer is C. The author’s proposal is actually extreme and violent, not gentle as the title says.
Satire, similar to irony, is achieved when you are trying to mock something or someone, usually by exaggerating and writing the opposite of what is expected. So, the title A Modest Proposal is not modest by any means - Swift wrote in this text that poor people should sell their children to rich people as food in order to keep the economy going. You can clearly see how the title is satirical then.