Final answer:
The Second Vatican Council was the event in the twentieth century that significantly promoted Christian unity by fostering dialogue and understanding among different Christian denominations.
Step-by-step explanation:
An important force for Christian unity in the twentieth century was C) The Second Vatican Council. The Great Schism, the Protestant Reformation, and the Council of Nicaea were significant events in the history of Christianity that resulted in fragmentation and division within the church. The Second Vatican Council, however, was convened by Pope John XXIII in the 1960s and is known for its efforts in promoting ecumenism and building bridges between different Christian denominations. This council significantly changed the Roman Catholic Church's relationship with other Christian communities, fostering greater unity among Christians.