Explanation

The famous ancient Chinese treatise 'The Art of War' was written by Sun Tzu, who is not among the options. Of the given authors, Niccolò Machiavelli wrote 'Dell'arte della guerra' (On the Art of War), published in 1521 — the only option who authored a work of that title, making A the intended answer. Thucydides wrote the History of the Peloponnesian War, Hobbes the Leviathan, and J.S. Mill On Liberty. Published solutions of this paper confirm Machiavelli as the key.