Stanislavsky's "Magic If" Refers To

Stanislavsky's "magic if" refers to:

A) A training method through which the performer could increase their physical dexterity and agility
B) A way through which the performer may imaginatively project themselves into the world of the play
C) The standard stage movement practice of performers at the Moscow Art Theatre, wherein the secondary actor moved to the center of the stage if the primary actor moved away from it
D) The practice of observing real life behavior and then transferring it directly to the stage