Correct answer: Martin Luther.
Martin Luther was not the first to attempt reform in the church. John Wycliffe in England and John Hus in Bohemia were important predecessors of the Reformation. But their work had less impact than Luther's. The Roman Catholic Church was strong enough in that earlier century, however, to block their work. Hus, for instance, was condemned by the Council of Constance in 1415 and burned at the stake.
Rome was less able to stamp out a heretic when Martin Luther started his movement in Germany in 1517. The politics of the time had something to do with that. Luther's prince in Saxony, known as Frederick the Wise, protected Luther and kept him alive when his life was in danger. Luther was the most prominent of the reformers who launched the Protestant Reformation. Ulrich Zwingli in Switzerland and John Calvin, active in France and Switzerland, both contributed much to the Reformation as well. But Luther is generally given the credit as the one who began the Reformation.