What is the primary characteristic of the very massive black holes found in active galactic nuclei?

The primary characteristic of the very massive black holes found in active galactic nuclei is their extraordinary mass, typically millions to billions of times the mass of the Sun. These supermassive black holes are located at the centers of galaxies and are essential to the dynamics of their host galaxies, influencing their formation and evolution.

The immense gravitational pull of these black holes allows them to attract substantial amounts of gas and dust, leading to phenomena such as the emission of energetic radiation when matter is accreted. This activity can result in the production of quasars and active galactic nuclei that are visible from vast distances. Their significant mass distinguishes them from stellar black holes, which form from the collapse of individual massive stars.

While collapsing stars are indeed a source of smaller black holes, they do not account for the supermassive black holes found in the centers of galaxies. The notion that they are small and localized doesn't apply either, as their effects extend across the entire galaxy. Though black holes do not emit light themselves, they can attract light from surrounding matter, hence the phrasing in the last option does not accurately capture their nature.