Based on the documentary you just watched, which significant event in Leonardo da Vinci's artistic career is estimated to have begun around 1503?
A) He moved to Rome to work for the Pope.
B) He began painting the Mona Lisa.
C) He completed 'The Last Supper' fresco.
D) He designed his famous flying machine concept.
E) He started his apprenticeship with Andrea del Verrocchio.