The second half of the sentence states that the prime minister has kept his promise to cultivate a certain leadership style; the “by” that begins the first part of the sentence indicates that the actions it describes are cited as examples of this style of leadership. The blank, then, must be filled with a word that describes the prime minister’s focus on recognizing commonalities and promoting collaboration. “Cooperation” does this. None of the other choices touches upon the prime minister’s efforts to work with others in the political process; indeed, “ideology” and “differentiation” might well imply the opposite.
Thus the correct answer is cooperation (Choice D).