Answer:
He used Prussia's army to defeat Austria and France, unifying the German states.
Step-by-step explanation:
France was against German reunification. A unified Germany posed a threat to France (seeing as the countries would share a border) and would upset the European balance of power. Under almost no circumstances would France have been willing to form an alliance to combine their armies with Prussia with the goal of German unification. The first answer is false.
Wilhelm I was the King of Prussia, but also the first Emperor of Germany, which means he had to be around post-unification. Being overthrown tended to mean execution in those days, so it isn't the second answer.
Helping Austria, a German-speaking region, win independence from the German Confederation would go directly against the notion of German unification. The third answer is false.
This leaves the fourth answer. There was a saying in Bismarck's time that most countries had an army, but Prussia was an army that had a country. However, it was not powerful enough to oppose all the other German states, the French, and the Austrians, especially considering the other German states would not appreciate being invaded and the other two countries wage war to prevent German unification. To solve this, Bismarck engaged in some 'Realpolitik' and created two common (and powerful) enemies of the German Confederation, France and Austria, to help "convince" the other German states to join and form a single country. As the other states would not be able to stand alone against a French or Austrian invasion, they had little choice but to join Prussia and help form the new, unified German Empire.