Answer:
The correct answer is option A. constellation.
Step-by-step explanation:
An apositive is a noun or nominal phrase positioned next to another noun, which changes its name or describes it in a different way.
It can be a simple word or some longer phrase.
In general, the information provided by an apositive could be deleted without affecting the meaning.
In this case, one could say "Sagittarius is near the Galactic Center of the Milky Way." without using the word constellation and the sentence would still mean the same.