If by curves you means the spiral arms, then none of those test answers are correct. the reason galaxies have concentrations in their arms is due to a "density wave" concentration of stars as they individually orbit the center of the galaxy. The spiral arms do not in fact rotate the same rate with the stars that constitute them, but are more like ripples in water- the peaks of a wave pattern in water equates to the spiral arms density.
Say our star was viewed from above as it orbited the galaxy, it would appear to approach a galactic arm, and the stars around it would come closer together somewhat as it neared the wave, and would then spread back apart as it passed the arm. Thus the arm is a wave peak of stars that slowly reveal its pattern in spiraling forms of density waves: spiral arms.
So the answer to your question would be:
f) density wave concentrations of our galactic core revealed through the concentrated density of a star "medium"
Another tangent of that question is "what is up with stars' strange orbital speed curve" which is a better way to word the question. What this means is that normally, the farther from the core you get, the slower the orbits should be (like the planets in our solar system do), but through study and measurement, they all appear to orbit in unison, which means the outer edge is moving much faster than the inner ones in order to keep up its position. Its odd, and the only theory that fits (its a theory, not an explanation, by the way) is a much larger mass around the edges of the galactic plane, acting like the rubber part of a bicycle wheel- the spokes are attached to the rim, so the spoke orbital curve matches that of a galaxys' stars.
Since this theory- this pattern that fits- seems to match the observed phenominon, it is guessed that there is a massive halo of matter, much more than what makes the stars up themselves, out there orbiting the disk like the rubber part of a tire. since we cant see the matter, nor detect it, we call it Dark Matter.
If THAT is what the obscurely worded question meant, then its A.