Question

sagittarius a* is
the site of the supermassive black hole at the center of the milky way.
the only confirmed binary black hole system.
the site of the supermassive black hole at the edge of our galaxy.
the source of the cosmic microwave background.

Explanation:

Sagittarius A* is the supermassive black - hole at the center of the Milky Way. It is not a binary black - hole system, not at the edge of the galaxy, and not the source of the cosmic microwave background.

Answer:

the site of the supermassive black hole at the center of the Milky Way.