How big is the Andromeda galaxy in relation to the Milky Way?

In terms of physical extent, the Andromeda Galaxy's disk diameter is well-measured, and spans 220,000 light-years across. For comparison, the diameter of the Milky Way's disk has long been thought to be only about half that: around 100,000 light-years.

i also think the correct answer here would be b.

we need more information in order to answer.