The answer is C. Astrophysicists now have "a template with which to make viable extrapolations concerning the structure and dynamics of many other galaxies." This is a very good thing! Because of that fact, astrophysicists are happy, or heartened by the fact that the Milky Way is so normal.

The key is that word "since," which creates a connection of causation between the two parts of the sentence. Because of the fact that astrophysicists can learn so much about other galaxies, they are heartened by the fact that the Milky Way is unremarkable.

The fact that astrophysicists can learn so much about other galaxies by studying the Milky Way is not a reason to distrust the fact that the Milky Way is an ordinary galaxy. So that eliminates A.