Final answer:
In the unified model of active galactic nuclei, the broad line region is the area close to the black hole's event horizon, marked by broad spectral lines due to high-velocity gas affected by the black hole's gravity.
Step-by-step explanation:
The question concerns the nature of the broad line region in active galactic nuclei (AGN). The correct answer is d) The region closest to the event horizon of a black hole where broad spectral lines are observed. This region is characterized by fast-moving gas that is subjected to the strong gravitational forces of the supermassive black hole at the center of the AGN. The gas's high velocity results in a wide range of Doppler shifts of emitted radiation, causing the broadening of spectral lines. This broad line region is essential for understanding the dynamics near supermassive black holes and the masses of the galaxies they inhabit.