It is generally accepted that modern diplomacy practices the started in Italy. As early practices of diplomacy started in Italy, early theories of diplomacy were also formulated there. One of the earlier of these theories belongs to Nicollo Machiavelli. Machiavelli’s formulation of diplomacy is based on his experience of serving as diplomat of Florence, his home city, until the fall of the republic in Florence in 1512. During his career Machiavelli took part in the foreign relations of Florence with both other Italian city-states and foreign countries. His theory of diplomacy can be read through his works The Prince, The Discourses on the First Ten Books of Titus Livy and The History of Florence. Thereofre, the correct option is D.