How Do We Observe the Supermassive Black Hole at the Center

How do we observe the supermassive black hole at the center of our galaxy?

A) Directly. We can see the dark circle inside its event horizon.
B) Directly. We can see the light that the black hole emits.
C) Indirectly. We can observe its gravitational effects on the nearby gas and stars.
D) We cannot make any observations of the supermassive black hole at the center of our galaxy.