Which example best illustrates the process of natural selection?

a
Giraffe necks varied in length and all offspring had an equal chance of survival.
b
Giraffe necks varied in length and the shorter neck giraffes were at a disadvantage.
c
Giraffes with the short necks migrated to locations best suited for their neck lengths.
d
Giraffes stretched their necks to reach food, and resulting offspring had longer necks.