Final answer:
Frederick II sought peace through diplomatic channels in 1190 and negotiated the Treaty of Jaffa in 1219, achieving Christian control over Jerusalem without violence.
Step-by-step explanation:
The answer to the student's question is A) 1190, Sought peace through diplomatic channels. During the Fifth Crusade, Frederick II, having struggled to garner support for his campaign and facing several setbacks including excommunication by Pope Gregory IX, realized the futility of continued conflict. Facing significant challenges and low morale among the Crusaders, he decided to engage in negotiations with Sultan al-Kamil and successfully concluded the Treaty of Jaffa in 1219. This treaty granted the Crusaders control of a kingdom centered around Jerusalem and a thirty-year truce, achieving their aims without violence, an outcome seen as a capitulation by many on both sides yet had the momentous consequence of establishing a Christian rule over more territory in the region than in the preceding two generations.