Final answer:
The accretion disk is the most often seen feature of a black hole during its low state because it emits X-rays and light as it spirals into the black hole, whereas other characteristics like the event horizon and singularity are not directly observable.
Step-by-step explanation:
Among the features of a black hole, the accretion disk is most often seen during its low state. An accretion disk is the disk of gas and dust found orbiting around black holes when they are in binary systems and are close enough to their binary companions to draw off material. This disk becomes superheated as it spirals in towards the black hole, emitting X-rays and sometimes visible light, making it observable from Earth. While black holes themselves and singularities are invisible since no light can escape them, and the event horizon is also not directly observable, the accretion disk can be seen due to its interaction with the black hole's immense gravitational field.
Moreover, the accretion disk is responsible for channelling the outflow of hot material into narrow jets oriented perpendicular to the disk. These jets often extend far from the black hole and can be traced back to within a few light-years of the source. As the matter within these jets moves close to the speed of light, they are a significant feature for studying black holes and quasars. The other features like the event horizon and singularity are hidden within the black hole, beyond our direct observation capabilities. Thus, the accretion disk, with its dynamic interactions and visibility, stands out as the most often observed feature during a black hole's low state.