What Is the Best Alternative Explanation for the Dark Matter

What is the best alternative explanation for the dark matter in the Milky Way based on the observed orbital motions of stars and gas?

A) We are not observing all the visible matter in the Milky Way.
B) We are not measuring the distances to gas clouds and stars properly.
C) We are not measuring the orbital velocities of gas clouds and stars properly.
D) We are not attributing enough mass to the visible matter in the Milky Way.
E) There is something wrong with our understanding of how gravity works.