Question

astronomers believe there are supermassive black holes at the centers of galaxies because
observations show gas clouds disappearing as they cross the event horizon of the enormous black hole.
stellar motion there indicates there is more mass than can be accounted for with normal matter.
galactic centers of nearby galaxies are a strong source of gravitational waves, indicating high masses there.
distant background quasars are gravitationally lensed by these central black holes.

Explanation:

Astronomers infer supermassive black - holes at galaxy centers from stellar motion. The motion indicates more mass than normal matter can account for, which is a key evidence. Gas clouds disappearing at the event horizon is a direct observation but less common as evidence compared to stellar motion. Gravitational waves from early - galaxy centers and gravitational lensing of quasars are not the most common or primary reasons for believing in supermassive black - holes at galaxy centers.

Answer:

stellar motion there indicates there is more mass than can be accounted for with normal matter.