When is BR Out?

[Paul L]

From the Giants/Rockies game Tuesday night (bottom of fifth):
R2 (Rajai Davis), no outs. B2 (Omar Vizquel) bunts fair down the first base line. F3 (Todd Helton) fields ball about thirty feet in front of first base as R2 arrives at and (let’s say) rounds third. BR stops about thirty feet down the line and F3 advances on BR at a half-trot for a tagout. BR backs up for about ten feet, then turns around and runs into the Giants third-base dugout without being tagged.

Question: when is BR out?
a) the instant he begins to back up towards home;
b) as soon as he backs up three feet to avoid the tag;
c) immediately upon touching home plate;
d) immediately upon passing home plate or the third base line;
e) as soon as he leaves the thirteen foot dirt circle around home plate;
f) at the point that the umpire judges that BR has abandoned his running duties; or
g) upon entering the dugout.