Which best describes a walking bass line?

a) Melodic line played on bass guitar
b) Melodic line played on an upright bass guitar
c) Short repeated sequence of notes which can become a theme of a piece
d) Repeated harmonic line in the bass with notes from a set chord progression

I know for sure it is not A. I do, however, think it's D., but I'm not positive.
I also hope it was appropriate to put this question under 'Arts'.