Contrary to Medieval Catholic Doctrine, How Did Luther Believe That

Contrary to medieval Catholic doctrine, how did Luther believe that a Christian could attain righteousness, a right standing before God?

A) by demonstrating a heart-felt devotion to Christ, evidenced in pure acts of sacrifice
B) by acknowledging the truth of Protestant dogma, while rejecting Catholic teachings
C) by having faith in Christ's atoning sacrifice on the cross
D) by recognizing in one's heart that God loves the said person and then responding to that love with love for God
E) by listening to the inner voice, a voice only Christians can truly hear