Final answer:
An accretion disc around a massive black hole would appear as a bright rotating ring through a telescope, with observable jets and radio emissions from the heated material in the disc providing evidence of the black hole's presence.
Step-by-step explanation:
An accretion disc around an extremely massive black hole would appear to a telescope as a bright, rotating ring of gas and dust, with different parts of the disc exhibiting varying levels of brightness due to the Doppler effect. If close enough, a telescope like the Hubble Space Telescope, with superior resolution, might observe the rotation of the accretion disc and map out jets of material being ejected from the vicinity of the black hole, as seen in galaxy NGC 4261. The hot material within the thin or "fat" accretion disc heats up as it spirals towards the black hole, potentially reaching temperatures of millions of degrees, thereby emitting a substantial amount of energy across various wavelengths.
To demonstrate the presence of a black hole at a galaxy's center, one must show that a substantial mass is compressed into a small volume, with no other known objects accounting for this phenomenon. Radio astronomy provides evidence by showing radio emissions from the heated accretion disc surrounding the black hole, such as the measurements from the Very Long Baseline Array detecting the compact radio source Sagittarius A* at the Milky Way's center.